Atmospheric deposition contributes little nutrient and sediment to stream flow from an agricultural watershed

Atmospheric deposition of nutrients within agricultural watersheds has received scant attention and is poorly understood compared to nutrient transport in surface and subsurface water flow pathways. Thus, we determined the deposition of phosphorus (P), nitrogen (N), and sediment in a mixed land use watershed in south-central Pennsylvania (39.5 ha; 50% corn–wheat–soybean rotation, 20% pasture, and 30% woodland), in comparison with stream loads at several locations along its reach between 2004 and 2006. There was a significant difference in deposition rates among land uses (P < 0.05) with more P and N deposited on cropland (1.93 kg P and 10.71 kg N ha⁻¹ yr⁻¹) than pasture (1.10 kg P and 8.06 kg N ha⁻¹ yr⁻¹) and woodland (0.36 and 2.33 kg N ha⁻¹ yr⁻¹). Although not significant, sediment showed the same trends among land uses. A significant relationship was found between P in deposition and P in soil <10-m away from the samplers suggesting much of the deposited sample was derived from local soil. Samplers adjacent to the stream channel showed deposition rates (1.64 kg P and 8.83 kg N ha⁻¹ yr⁻¹) similar to those on cropland. However, accounting for the surface area of the stream, direct deposition of P, N, and sediment probably accounted for <3% of P and <1% of N and sediment load in stream flow from the watershed (1.41 kg P, 27.09 kg N, and 1343 kg sediment ha⁻¹ yr⁻¹ at the outlet). This suggests that strategies to mitigate nutrient and sediment loss in this mixed-land use watershed should focus on runoff pathways.     

Streamwater phosphorus and nitrogen across a gradient in rural–agricultural land use intensity

This paper provides an overview of the impacts of rural land use on lowland streamwater phosphorus (P) and nitrogen (N) concentrations and P loads and sources in lowland streams. Based on weekly water quality monitoring, the impacts of agriculture on streamwater P and N hydrochemistry were examined along a gradient of rural–agricultural land use, by monitoring three sets of ‘paired’ (near-adjacent) rural headwater streams, draining catchments which are representative of the major geology, soil types and rural/agricultural land use types of large areas of lowland Britain. The magnitude and timing of P and N inputs were assessed and the load apportionment model (LAM) was applied to quantify ‘continuous’ (point) source and ‘flow-dependent’ (diffuse) source contributions of P to these headwater streams. The results show that intensive arable farming had only a comparatively small impact on streamwater total phosphorus (TP loads), with highly consistent stream diffuse-source TP yields of ca. 0.5 kg-P ha^?1 year^?1 for the predominantly arable catchments with both clay and loam soils, compared with 0.4 kg-P ha^?1 year^?1 for low agricultural intensity grassland/woodland on similar soil types. In contrast, intensive livestock farming on heavy clay soils resulted in dramatically higher stream diffuse-source TP yields of 2 kg-P ha^?1 year^?1. The streamwater hydrochemistry of the livestock-dominated catchment was characterised by high concentrations of organic P, C and N fractions, associated with manure and slurry sources. Across the study sites, the impacts of human settlement were clearly identifiable with effluent inputs from septic tanks and sewage treatment works resulting in large-scale increases in soluble reactive phosphorus (SRP) loads and concentrations. At sites heavily impacted by rural settlements, SRP concentrations under baseflow conditions reached several hundred μg-P L^?1. Load apportionment modelling demonstrated significant ‘point-source’ P inputs to the streams even where there were no sewage treatment works within the upstream catchment. This indicates that, even in sparsely populated rural headwater catchments, small settlements and even isolated groups of houses are sufficient to cause significant nutrient pollution and that septic tank systems serving these rural communities are actually operating as multiple point sources, rather than a diffuse input.     

Influence of no-tillage on the distribution and lability of phosphorus in Finnish clay soils

No-tillage (NT) is a method adopted to reduce erosion and particulate phosphorus (P) load from arable land to watercourses. However, it has been found to increase the loss of dissolved P with surface runoff, but the reasons for that have rarely been examined in detail. The objective of the present study was to determine the chemical factors explaining this response by investigating the impact of NT on the type and distribution of P reserves as well as on organic carbon (C) in the 0–35 cm topsoil layer of clay soil profiles (Vertic Cambisols). Soil samples were taken from two experimental fields (Jokioinen and Aurajoki) at 0–5, 5–20 and 20–35 cm depths in conventionally tilled (CT) and non-tilled (for 4–5 years) plots. The plots had been cultivated and fertilized according to the common field practices in Finland (15–18 kg P and 100–128 kg N ha⁻¹ year⁻¹).Inorganic and organic P reserves characterized by a modified Chang and Jackson fractionation procedure were not significantly affected by the cultivation methods. However, in the uppermost soil layer (0–5 cm) in NT of the Jokioinen field, the labile P determined by water extraction (P_w) increased significantly, whereas the increase in P extracted with acid ammonium acetate (P_AAC) remained statistically insignificant. The increase in labile P coincided with a significant increase in organic carbon (C), which supports the theory that competition between organic anions and phosphate for the same sorption sites on oxide surfaces will enhance the lability of soil P. In the Aurajoki field with distinct soil cracking, P_w and P_AAC were not affected by NT in the uppermost soil layer, but they increased in the deepest soil layer (20–35 cm) concomitantly with an increase in Al-bound P and organic C. However, the increases were not statistically significant. In both fields, soil acidification due to the repeated application of N fertilizers at a shallow soil depth as well as the accumulation of organic C lowered pH of the uppermost soil layer in NT compared to the deeper soil layers. The results indicated that even short-term NT can increase the labile P in clay soil. However, further studies are needed to assess the long-term changes in lability of surface soil P and, consequently, the possible need for readjustment of the fertilization level in NT.     

Managing erosion and water quality in agricultural watersheds by small detention ponds

Terrace-contouring systems with on-site water detention cannot be installed in areas of complex topography, small parceling and multi-blade moldboard plow use. However, field borders at the downslope end may be raised at the deepest part where runoff overtops to create detention ponds, which can be drained by subsurface tile outlets and act similar to terrace-contouring systems. Four of such detention ponds were monitored over 8 years. Monitored effects included the prevention of linear erosion down slope, the sediment trapping from upslope, the enrichment of major nutrients in the trapped and delivered sediments, the amount of runoff retained temporarily, the amount of runoff reduced by infiltration, the decrease in peak runoff rate and the decrease in peak concentrations of agrochemicals due to the mixing of different volumes of water within the detention ponds. The detention ponds had a volume of 30–260 m³ ha⁻¹ and trapped 54–85% of the incoming sediment, which was insignificantly to slightly depleted (5–25%) in organic carbon, phosphorus, nitrogen and clay as compared to the eroding topsoil, while the delivered sediment was strongly enriched (+70–270%) but part of this enrichment already resulted from the enrichment of soil loss. The detention ponds temporarily stored 200–500 m³ of runoff. A failure was never experienced. Due to the siltation of the pond bottom, the short filled time (1–5 days) and the small water covered area, infiltration and evaporation reduced runoff by less than 10% for large events. Peak runoff during heavy rains was lowered by a factor of three. Peak concentrations of agrochemicals (Terbutylazin) were lowered by a factor of two. The detention ponds created by raising the downslope field borders at the pour point efficiently reduced adverse erosion effects downslope the eroding site. They are cheap and can easily be created with on-farm machinery. Their efficiency is improved where they are combined with an on-site erosion control like mulch tillage because sediment and runoff input are reduced. Ponds had to be dredged only after the first year when on-site erosion control was not fully effective.     

坡耕地薄层紫色土-岩石系统中氮磷的迁移特征

于2015年两场代表性降雨事件下研究了紫色土坡耕地(1 500 m~2)水文过程驱动的氮磷地表和地下迁移特征.结果表明:(1)前期土壤水饱和度和降雨量决定地表径流和裂隙潜流的产流深度和胶体浓度的基本水平;降雨强度则控制产流量的变化趋势和胶体浓度的峰值水平.(2)氮素主要以溶解态随裂隙潜流迁移;磷素则主要以细颗粒结合态通过地表径流流失.(3)氮磷迁移过程的水文驱动特征明显,氮素迁移主要受裂隙潜流的驱动作用影响,而磷素迁移则主要受地表径流的驱动作用影响.该研究结果丰富了对坡耕地薄层土壤-裂隙母岩二元系统中氮磷迁移对降雨响应规律的认识,可为农业氮磷非点源污染防治提供可靠的田间数据支持并明确关键的水文调控环节.     

Linkages between land management activities and water quality in an intensively farmed catchment in southern New Zealand

Linkages between land management activities and stream water quality are reported for a 2480 ha catchment used for dairy farming, sheep–beef farming and forestry in Southland, New Zealand. Our approach was to reconcile measured loads of nutrients exported from the catchment with those estimated based on characterisation of farming practices within the catchment. The latter was based upon detailed surveys of farm practices and soil quality. Monthly stream monitoring showed that median nutrient (N and P), sediment and faecal bacteria concentrations exceeded guidelines recommended for surface waters. Measured specific yields for suspended sediment (SS), total N (TN) and phosphorus (P) discharged from the catchment were 58, 8.2 and 0.43 kg ha⁻¹ year⁻¹, respectively, for the 2001–2005 monitoring period. In comparison, model estimates of N and P losses in drainage and overland flow from farms in the catchment were 10.1 and 0.59 kg ha⁻¹ year⁻¹, respectively. Field measurements, farm management surveys and farm systems modeling have identified some land management practices that appear to be key sources of many of these pollutants. These sources include subsurface drainage systems (including the preferential flow of irrigated effluent through these soils), overland flow from the heavy soils used for dairy farming in the catchment and the practice of intensively wintering cows on forage crops. Modeling suggests that a significant improvement in catchment water quality could be achieved through the implementation of targeted best management practices (BMPs) on dairy farms in the catchment. These include (i) covered feedpad wintering systems for controlling N losses, (ii) nitrification inhibitor use on milking platforms, (iii) deferred irrigation and low rate application of farm dairy effluent and (iv) limiting soil Olsen P to economically optimum levels. The adoption of these BMPs will, in part, depend on their economic viability. This paper therefore presents a double-bottom-line analysis (i.e. environmental and economic) of some of these BMPs and discusses their potential to cost-effectively deliver improved water quality in the Bog Burn catchment.     

Modelling nitrogen cycles of farming systems as basis of site- and farm-specific nitrogen management

The paper describes a model designed for analysing interrelated nitrogen (N) fluxes in farming systems. It combines the partial N balance, farm gate balance, barn balance and soil surface balance, in order to analyse all relevant N fluxes between the subsystems soil–plant–animal–environment and to reflect conclusive and consistent management systems. Such a system approach allows identifying the causes of varying N surplus and N utilisation.The REPRO model has been applied in the experimental farm Scheyern in southern Germany, which had been subdivided into an organic (org) and a conventional (con) farming system in 1992. Detailed series of long-term measuring data are available for the experimental farm, which have been used for evaluating the software for its efficiency and applicability under very different management, yet nearly equal site conditions.The organic farm is multi-structured with a legume-based crop rotation (N₂ fixation: 83 kg ha⁻¹ yr⁻¹). The livestock density is 1.4 LSU ha⁻¹. The farm is oriented on closed mass cycles.The conventional farm is a simple-structured cash crop system based on mineral N (N input 145 kg ha⁻¹ yr⁻¹). Averaging the years 1999–2002, the organic crop rotation reached, with regard to the harvested products, about 81% (6.9 Mg ha⁻¹ yr⁻¹) of the DM yield and about 93% (140 kg ha⁻¹ yr⁻¹) of the N removal of the conventional rotation. Related to the cropped area, the N surplus calculated for the organic rotation was 38 kg ha⁻¹ yr⁻¹ versus 44 kg ha⁻¹ yr⁻¹ for the conventional rotation. The N utilisation reached 0.77 (org) and 0.79 (con), respectively. The different structure of the farms favoured an enhancement of the soil organic nitrogen stock (35 kg ha⁻¹ yr⁻¹) in the organic crop rotation and caused a decline in the conventional system (−24 kg ha⁻¹ yr⁻¹). Taking account of these changes, which were substantiated by measurements, N surplus in the organic rotation decreased to 3 kg ha⁻¹ yr⁻¹, while it increased to 68 kg ha⁻¹ yr⁻¹ in the conventional system. The adjusted N utilisation value amounted to 0.98 (org) and 0.69 (con), respectively.     

Spatial and long-term variability of soil loss due to crop harvesting and the importance relative to water erosion: A case study from Belgium

The harvest of crops such as sugar beet (Beta vulgaris L.), potato (Solanum tuberosum L.), leek (Allium porrum L.) and carrot (Daucus carota L.) causes soil loss from arable land because soil adhering to the crop and soil clods that failed to be separated by the harvesting machine, are exported from the field together with these harvested crops. These soil losses can be of the same order of magnitude as soil losses caused by water erosion processes, but are often neglected in soil erosion research. In this article we developed a methodology to investigate the spatial and long-term (1846–2004) variability of soil loss due to crop harvesting (SLCH) in Belgium and the spatial distribution of the importance of SLCH relative to soil losses caused by water erosion processes in Flanders. The study is based on long-term time series of soil tare data of crop processing factories and area and crop yield statistics. Until the middle of the 20th century, potato and roots and tubers grown as second crop, had the largest share in the SLCH-crop growing area in Belgium. Sugar beet gained importance from the end of the 19th century onwards and has now, of all SLCH crops, the largest growing area. We could estimate that, partly due to increasing crop yields and the mechanisation of the harvesting process, SLCH per hectare of cropland increased from 0.4 Mg ha⁻¹ year⁻¹ in 1846 to 2.4 Mg ha⁻¹ year⁻¹ in the 1970s and early 1980s. Since then mean annual soil losses decreased again to 1.8 Mg ha⁻¹ year⁻¹ in 2004. It was assessed that total yearly SLCH in Belgium rose from more than 575,000 Mg in the middle of the 19th century to more than 1.7 × 10⁶ Mg in the 1970s and early 1980s, while current SLCH values are 1.4 × 10⁶ Mg. We estimated that since 1846, more than 163 × 10⁶ Mg soil was exported from cropland in Belgium through this erosion process, which corresponds to 109 hm³ or an average soil profile truncation of 1.15 cm. Average sediment export from cropland in Flanders was 3.7 Mg ha⁻¹ year⁻¹ in 2002, of which 46% was due to SLCH and 54% was due to water erosion processes. The relative importance of SLCH varied, depending on the agricultural region, between 38% and 94%.     

Runoff and sediment losses from 27 upland catchments in Southeast Asia: Impact of rapid land use changes and conservation practices

Rapid changes in upland farming systems in Southeast Asia generated predominantly by increased population pressure and ‘market forces’ have resulted in widespread land degradation that has been well documented at the plot scale. Yet, the links between agricultural activities in the uplands and downstream off-site effects remain largely unknown because of the difficulties in transferring results from plots to a larger scale. Many authors have thus pointed out the need for long-term catchment studies. The objective of this paper is to summarize the results obtained by the Management of Soil Erosion Consortium (MSEC) over the last 5 years from 27 catchments in five countries (Indonesia, Laos, Philippines, Thailand, and Vietnam). The purpose of the study was to assess the impacts of cultivation practices on annual runoff and erosion rates. Initial surveys in each catchment included topography, soils and land use. Monitoring included climatic, hydrologic and erosion (total sediment yield including bed load and suspended sediment load) data, land use and crop yields, and farmers’ income. In addition, new land management options were introduced through consultations with farmers and evaluated in terms of runoff and erosion. These included tree plantations, fruit trees, improved fallow with legumes, maize intercropped with legumes, planted fodder, native grass strips and agro-ecological practices (direct sowing and mulch-based conservation agriculture). Regressions analyses showed that runoff during the rainy season, and normalized runoff flow coefficient based on erosive rainfall during the rainy season (rainfall with intensity exceeding 25 mm h⁻¹) increase with the percentage of the catchment covered by maize. Both variables decrease with increasing soil depth, standard deviation of catchment slope (that reflects terrain roughness), and the percentages of the catchment covered by fallow (regular and improved), tree plantations and planted fodder. The best predictors of sediment yield were the surface percentages of maize, Job's tears, cassava and footpaths. The main conclusions generated from this study were: (i) soil erosion is predominantly influenced by land use rather than environmental characteristics not only at the plot scale but also at the catchment scale; (ii) slash-and-burn shifting cultivation with sufficiently long rotations (1 year of cultivation, 8 years of fallow) is too often unjustly blamed for degradation; (iii) in its place, continuous cropping of maize and cassava promotes high rates of soil erosion at the catchment scale; (iv) conservation technologies are efficient in reducing runoff and total sediment yield at the catchment scale; (v) the adoption of improved soil management technologies by upland farmers is not a function of the degree of intensification of their farming system and/or of their incomes. The results suggest that if expansion of maize and cassava into already degraded upland systems were to occur due to increased demand for biofuels, there is a risk of higher runoff and sediment generation. A failure to adopt appropriate land use management strategies will result in further rapid resource degradation with negative impacts to downstream communities.     

Importance of indirect nitrous oxide emissions at the field, farm and catchment scale

Direct and indirect nitrous oxide (N₂O) emissions and leaching losses from an intensively managed grazed pasture in the Ythan catchment, Aberdeenshire, UK, were measured and compared over a 17-month period. Simultaneous measurements of farm-wide leaching losses of N₂O were also made and catchment-wide fluxes were estimated from existing N leaching data. The relative importance of direct and indirect N₂O fluxes at the field, farm and catchment scale was then assessed. At the field scale we found that direct N₂O emissions were low (1.2 kg N ha⁻¹ year⁻¹, 0.6% of N input) with indirect N₂O emissions via drainage waters comprising a significant proportion (25%) of total N₂O emissions. At the whole-farm scale, the N₂O-N emission factor (0.003) for leached NO₃-N (EF_5-g) was in line with the IPCC's recent downward revision. At the catchment scale, a direct N₂O flux of 1.9 kg N ha⁻¹ year⁻¹ and an indirect flux of 0.06 kg N₂O-N ha⁻¹ year⁻¹ were estimated. This study lends further support to the recent downward revision of the IPCC emission factor for N₂O arising from leached N in surface and ground waters (EF_5-g) and highlights the need for multiple point sampling to ensure that the importance of indirect N₂O losses via drainage waters is not misrepresented at the farm and catchment scales.     

华北半干旱区2个农业流域地表氮素流失特征的对比研究

在2004和2005年2个水文年期间,通过野外降雨径流监测和室内分析,对位于华北半干旱地区天津市重要饮用水水源地于桥水库周边2个典型农业小流域地表径流氮素流失特征进行了对比研究.结果表明,2个农业小流域全年氮输出主要集 中在6-9月;试验期间桃花寺流域41%的地表径流和52%的总氮负荷发生在2次>60 mm降雨中,曹各庄流域输出地表径 流和总氮负荷较均匀地分布于不同降雨类型的降雨事件中;桃花寺流域和曹各庄流域地表径流氮素输出发生的最小降雨量 分别约为20 mm和10 mm;曹各庄流域和桃花寺流域年均径流系数分别为0.013 2和0.001 6,前者为后者的8.3倍;曹各庄流域 年均总氮输出量是1.048 kg·(hm2·a)-1,桃花寺流域年均总氮输出量是0.158 kg·(hm2·a)-1,前者是后者的6.6倍.2个农业流 域内的微地形地貌、景观格局和迁移廊道性质的不同是造成流域氮素流失特征差异的主要原因:受长期人为活动干扰形成的 复杂地形地貌结构降低了流域的降雨一产流能力;桃花寺流域地表径流氮素主要污染源(村庄)离受纳水体的距离约为 1 500 m,而曹各庄流域此距离约为200 m,较短的迁移距离增加氮素迁移到受纳水体的风险;曹各庄流域道路型的传输廊道可 以很快地将地表径流氮素迁移到受纳水体,而桃花寺流域地表径流氮素则可以在传输廊道中被小石坝、植被过滤带和干塘等 "汇"型结构所持留.     

Rice mill waste as organic manure on a degraded Alfisol

In much of sub-Saharan Africa (SSA), the lack of organic soil amendments constitutes one of the principal causes for declining soil fertility in intensifying farming systems. The challenge, therefore, remains to increase the availability of organic inputs and to develop recommendations for their combination with inorganic fertilizers. An on-farm experiment was conducted in the northern Guinea savanna of Nigeria to evaluate the fertilizer effect of rice (Oryza sativa L.) mill waste (RMW) on a degraded Alfisol. The decomposition and nutrient (N and P) release patterns of RMW were studied using the litterbag technique, and the effect on maize yield and soil properties was determined. The RMW was applied at rates of 0, 5, 10, and 15 Mg DM ha⁻¹ and was applied either unburnt or burnt (farmers’ practice). In both years, compound fertilizer was broadcast during land preparation on all plots at rates of 40 kg N ha⁻¹, 17 kg P ha⁻¹, and 33 kg K ha⁻¹.Results obtained in the litterbag study showed that, at maize harvest, more than 90% of the P had been released from the decomposing RMW. However, around 60% of the organic C and 45% of the N still remained. Compared to the control treatment (0 Mg ha⁻¹ RMW), which yielded 0.55 Mg ha⁻¹, maize (Zea mays L.) grain yields were increased by 95% when 10 Mg ha⁻¹ of unburnt RMW was applied, and by 147% with 15 Mg ha⁻¹ (mean of 2 years). In contrast, burnt RMW did not result in significant yield increases. The cumulative application of 30 Mg ha⁻¹ of unburnt RMW significantly increased the soil organic carbon in the surface soil from 0.7% (0 Mg ha⁻¹ RMW) to 1.3%.The results of this investigation indicate that RMW constitutes a valuable organic input in the Guinea savanna if applied unburnt at rates of 10–15 Mg ha⁻¹ in combination with inorganic fertilizer. The repeated application of unburnt RMW may contribute to the rehabilitation of degraded soils through the buildup of soil organic matter.     

Simulated dynamics of carbon stocks driven by changes in land use, management and climate in a tropical moist ecosystem of Ghana

Sub-Saharan Africa is large and diverse with regions of food insecurity and high vulnerability to climate change. This project quantifies carbon stocks and fluxes in the humid forest zone of Ghana, as a part of an assessment in West Africa. The General Ensemble biogeochemical Modeling System (GEMS) was used to simulate the responses of natural and managed systems to projected scenarios of changes in climate, land use and cover, and nitrogen fertilization in the Assin district of Ghana. Model inputs included historical land use and cover data, historical climate records and projected climate changes, and national management inventories. Our results show that deforestation for crop production led to a loss of soil organic carbon (SOC) by 33% from 1900 to 2000. The results also show that the trend of carbon emissions from cropland in the 20th century will continue through the 21st century and will be increased under the projected warming and drying scenarios. Nitrogen (N) fertilization in agricultural systems could offset SOC loss by 6% with 30 kg N ha⁻¹ year⁻¹ and by 11% with 60 kg N ha⁻¹ year⁻¹. To increase N fertilizer input would be one of the vital adaptive measures to ensure food security and maintain agricultural sustainability through the 21st century.     

Runoff water quality from manured riparian grasslands with contrasting drainage and simulated grazing pressure

Globally, management of grazed riparian areas is critically important to agricultural sustainability and environmental quality. However, the potential impacts of riparian grazing management on water quality are not well-documented, particularly in the southeastern USA. The objective of this work was to determine sediment and nutrient export under simulated rainfall from poorly drained and well-drained riparian soils where heavy or light grazing pressure by cattle was simulated. Plots were established on stands of existing vegetation to create grazing pressure treatments of (a) light-use (full ground cover, uncompacted), and with stands modified to establish (b) heavy-use (bare ground, compacted) treatments. Vegetation on poorly drained soils consisted of several typical wetland species (e.g., Pontederia cordata L., Juncus coriaceus Mackenzie) in the southeastern USA, whereas mixed tall fescue (Festuca arundinacea Schreb.)–dallisgrass (Paspalum dilatatum Poir.) stands were the dominant vegetation on well-drained soils. Runoff volume was generally greater from heavy-use than from light-use for poorly drained soils and for well-drained soils. Greater runoff volume was also observed from poorly drained soils compared to well-drained soils for both light-use and for heavy-use treatments. Light-use plots were remarkably effective at minimizing export of total suspended solids (TSS) on both soils (<30 kg ha⁻¹). Mean total Kjeldahl P (TKP) export was fourfold greater from heavy-use plots than from light-use plots on both soils. While export of nitrate-nitrogen (NO₃-N) was unaffected by grazing pressure and soil drainage, mean ammonium-nitrogen (NH₄-N) and total N (TN) export from poorly drained heavy-use plots was greater than fivefold that from well-drained light-use plots. Results indicate that livestock heavy-use areas in the riparian zone may export substantial TSS and nutrients, especially on poorly drained soils. However, when full ground cover is maintained on well-drained soils, TSS and nutrient losses may be limited.     

Soil fertility dynamics in runoff-capture agriculture, Canary Islands, Spain

In the extremely arid (∼150 mm yr⁻¹) eastern Canary Islands of Lanzarote, Fuerteventura and La Graciosa, agriculture has been sustained for decades by a traditional runoff-capture (RC) farming system known as “gavias”. Although the main goal of these systems is to increase water supply for crops, making unnecessary conventional irrigation, a secondary and equally important factor is that this system allows for sustainable agricultural production without addition of chemical or organic fertilizers. A field study was conducted to assess the impact of long-term agriculture (>50 yr) on soil fertility and to evaluate key factors affecting the nutrient sustainability of RC agricultural production. Soil fertility and nutrient dynamics were studied through chemical characterization of the arable layer (0-25 cm) of RC agricultural plots, adjacent natural soils (control) not affected by runoff and cultivation, and sediments contributed by a series of RC events. Results showed that RC soils have enhanced fertility status, particularly because they are less affected by salinity and sodicity (mean electrical conductivity = 1.8 dS m⁻¹ vs. 51.0 dS m⁻¹ in control soils; mean exchangeable sodium percentage = 11.1% vs. 30.6% in control soils), and have higher water and nutrient holding capacities (mean clay plus silt contents ≈87% vs. 69% in control soils). In general, sediments transported with the runoff and deposited in RC plots (average sediment yield ≈ 46 ton ha⁻¹ yr⁻¹), contain sufficient nutrients to prevent a progressive reduction of essential plant nutrients below natural levels in spite of nutrient uptake and removal by the harvested crop. Average additions of nitrogen, phosphorus and potassium with the runoff sediments were 33.6, 35.3 and 48.8 kg ha⁻¹ yr⁻¹, respectively. Results of this study show how a crop production system can be sustained in the long term by natural hydrological and biogeochemical catchment processes. This system maintains a nutrient balance that is not based on energy-intensive inputs of fertilizers, but is integrated in natural nutrient cycling processes, unlike other tropical farming agroecosystems.     

不同雨强对太湖河网区河道入湖营养盐负荷影响

为揭示太湖河网区不同雨强下入湖河道面源污染规律,以太湖入湖负荷最大的河道大浦河为例,通过一周年逐日高频监测水体各形态氮、磷及溶解性有机碳等营养盐情况,结合河道流量及降雨量的自动监测资料,分析了大雨、中雨、小雨及无雨等4种降雨强度下太湖河网区典型河道的流量和营养盐负荷特征.结果表明,作为太湖河网区的典型入湖河道,大浦河时常发生往复流现象,观测的365 d内,有60 d日均流量为负值,占16%;河道流量对雨强的响应较为迟缓,仅强降雨事件( 25 mm·d~(-1))下,降雨当日河道流量才显著增加;中雨期平均流量仅比无雨期高了29%,在统计上不显著.河道水体营养盐浓度在不同降雨强度下差异不显著,大雨、中雨、小雨及无雨事件下河道总氮浓度分别为(3. 00±0. 58)、(3. 34±0. 93)、(3. 55±1. 05)和(3. 37±1. 14) mg·L~(-1),小雨事件当天水体总氮浓度均值最高,而4种类型降雨事件下河道总磷含量分别为(0. 228±0. 068)、(0. 258±0. 121)、(0. 219±0. 083)和(0. 225±0. 121) mg·L~(-1),差异性也不显著,就平均值而言,中雨时河道总磷浓度最高.夏季典型降雨过程分析表明,不同雨强发生之后河道溶解性有机碳和各形态氮的浓度变化不大,但大雨之后次日河道各形态磷浓度有明显增高,持续时间为2 d,中雨后次日河道总磷和颗粒态磷有明显增高,持续时间仅为1 d,小雨后磷浓度基本无变化.大雨、中雨、小雨和无雨时总氮日负荷分别为7. 64、3. 19、3. 21和2. 62 t·d~(-1),总磷日负荷分别为0. 59、0. 26、0. 22和0. 20 t·d~(-1),受入湖流量影响,大雨期营养盐日负荷显著高于中雨及以下强度的降雨;然而,由于一年内大雨出现频次较少,大雨期氮和磷总入湖负荷占年负荷的比重不大,大雨期总氮和总磷分别入湖61. 11 t和4. 72 t,占观测周年的5. 6%和5. 8%,这与山区河道降雨负荷间的关系有着显著区别.本高频观测表明,太湖流域平原河网区河道面源污染汇集过程复杂,入湖负荷受降雨强度的影响相对较小,入湖水量是营养盐负荷的重要影响因素.本研究结果对太湖流域平原河网区湖泊的面源污染的估算及控制对策的制定具有参考价值.     

Managing crop residues, fertilizers and leaf litters to improve soil C, nutrient balances, and the grain yield of rice and wheat cropping systems in Thailand and Australia

This paper reports on the influence that residue and fertilizer management have on nutrient balances, soil organic matter (SOM) dynamics, and crop yields of a flooded rice system in northeast Thailand (1992–1997) and a wheat–forage legume rotation in eastern Australia (1992–1998). Both soils had been subject to at least 18 years of cultivation and had lost up to 90% of the original labile (C_L) and 85% of the total carbon (C_T).For the rainfed rice cropping systems of northeast Thailand, a system is described in which small applications of leaf litter from locally grown trees are applied annually to rice paddy soils prior to transplanting. Annual applications of 1500 kg ha⁻¹ of leaf litter from different locally grown shrubs for five seasons resulted in increases in rice grain yield in 1997 of between 20 and 26% above the no-leaf litter control. Nutrient balances, determined by the difference between the inputs (fertilizer and added leaf litters) and outputs (grain and straw), indicated net positive balances of up to 457 kg N ha⁻¹, and 60 kg P ha⁻¹, after five seasons of leaf litter applications. Sulfur and potassium balances resulted in net deficits of up to −13 kg S ha⁻¹ and −52 kg P ha⁻¹, where no leaf litter was applied and rice straw was removed following harvest. Soil carbon (C) concentrations increased significantly only where higher fertilizer rate and rice stubble retention were combined.The poor management of fertilizers and crop residues, and excessive cultivation has also resulted in large soil fertility losses in the grain growing areas of Eastern Australia. After five wheat and two legume/fallow crops, negative N balances of up to −303 kg ha⁻¹ were calculated for the treatments where wheat stubble was not retained and bare fallow leys were used. The balance of nutrients such as K, which are contained in larger proportions in stubble, were found to be up to −362 kg ha⁻¹ on the straw-removed treatments and up to +29 kg ha⁻¹ on the straw-retained treatments. Forage legume leys resulted in short term increases in C_L and the carbon management index (CMI).Sustainable farming systems require that crop yields are stable through the maintenance of soil fertility and the balance of nutrients in the system. Increases in soil C levels require sustained periods of balanced fertilization and residue retention.     

近地表土壤水分条件对坡面农业非点源污染物运移的影响

采用人工模拟降雨实验,研究了近地表土壤水分条件,尤其是土壤水分饱和条件对土壤侵蚀过程中农业非点源污染物运移的影响.结果表明,前期近地表土壤水分条件对土壤侵蚀过程中农业非点源污染物的迁移有着很大影响.饱和含水量时径流及泥沙中非点源污染物的流失浓度和流失量大于非饱和含水量,且前期近地表土壤含水量越大,径流及泥沙中农业非点源污染物的流失浓度和流失量越大.土壤氮素的主要流失途径是降雨所产生的径流,约占总流失量的90.4%～99.8%;土壤磷素的流失途径是降雨径流和侵蚀产沙,分别占总流失量的2.67%～23.5%和76.5%～97.3%.同时,土壤质地对磷素养分的流失有很大影响,杨凌土随泥沙流失的DP浓度和流失量均大于安塞黄绵土.最后,提出了采取最佳管理措施等控制农业非点源污染的针对性建议.     

Testing the Norwegian phosphorus index at the field and subcatchment scale

Simple risk assessment tools for agricultural phosphorus (P) losses, like the P index, have been developed in the U.S.A. and in some European countries. Despite its popularity, there have been surprisingly few studies, which try to test the index close to the field scale. For Norway, the P index approach comprises the risk related to both the source of P (soil P status, amount of fertilizer and manure as well as method of application, plant P release by freezing and P balance) and the risk related to transport of P (erosion, flooding, surface runoff, contributing distance, modified connectivity, soil profile, subsurface drainage). In this paper, we have applied the Norwegian P index to farmer fields within a small agricultural catchment, the Skuterud catchment (450 ha), in southeastern Norway. The Norwegian P index was tested for two agricultural fields (0.3–0.4 ha) and nine subcatchments (6–65 ha). Total P concentrations in runoff from the 11 study areas were measured during the year from May 2001 to April 2002. Results from the testing showed that the Norwegian P index described 66% of the variation in measured relative total P concentration for fields and subcatchments included in this study. Additionally, the P index was able to detect fields and subcatchments with the highest measured P concentrations. Results also showed that the source factor contributed most to the variation between fields and hence were important for the identification of high-risk areas in Skuterud catchment. It was found that the soil P status described 66% of the variation in the source factor. Among the transport variables, it was found that both erosion risk and contributing distance had an important influence on the transport factor. Overall, the study illustrated the potential of the P index to detect areas with the highest risk of P loss.     

Runoff and dissolved organic carbon loss from a paired-watershed study of three adjacent agricultural Watersheds

Organic matter plays several important roles in the biogeochemistry of terrestrial and aquatic ecosystems including the mobilization and transport of nutrients and pollutants. Cropping, tillage practices and vegetative buffer strip installation affect losses of dissolved organic carbon (DOC). While many studies show reductions in pollutant export from agroecosystems where vegetative buffers have been implemented, buffer strips may be a source of DOC and contribute to surface water pollution. Using a paired-watershed approach, the objectives of this study were to determine the effect of grass and agroforestry buffers on runoff and DOC loss, compare runoff and DOC losses between the growing and fallow seasons, and investigate crop effects on runoff and DOC losses. The study design consisted of three small agricultural Watersheds in a no-till, maize-soybean rotation located in the claypan region of northeast Missouri, USA; one watershed was planted with grass buffer strips, one with agroforestry buffer strips, and one unaltered watershed served as the control. Runoff and DOC loss were measured during a six-year calibration period (1991–1997) prior to buffer installation and for a nine-year treatment period (1997–2006). The grass buffer strips significantly decreased runoff by 8.4% (p = 0.015) during the treatment period while the agroforestry buffer system exhibited no significant change in runoff (p = 0.207). Loss of DOC was not significantly affected by grass or agroforestry buffer installation (p = 0.535 and p = 0.246, respectively). Additionally, no significant difference in runoff or DOC loss was found between crops (maize and soybean) or between seasons (growing and fallow). Overall, this study indicates that grass buffer systems are effective at reducing runoff and that DOC contamination of surface waters is not exacerbated by either type of vegetative buffer strip.