Carbon dioxide efflux from soil with poultry litter applications in conventional and conservation tillage systems in northern Alabama

Increased CO2 release from soils resulting from agricultural practices such as tillage has generated concerns about contributions to global warming. Maintaining current levels of soil C and/or sequestering additional C in soils are important mechanisms to reduce CO2 in the atmosphere through production agriculture. We conducted a study in northern Alabama from 2003 to 2006 to measure CO2 efflux and C storage in long-term tilled and non-tilled cotton (Gossypium hirsutum L.) plots receiving poultry litter or ammonium nitrate (AN). Treatments were established in 1996 on a Decatur silt loam (clayey, kaolinitic thermic, Typic Paleudults) and consisted of conventional-tillage (CT), mulch-tillage (MT), and no-tillage (NT) systems with winter rye [Secale cereale (L.)] cover cropping and AN and poultry litter (PL) as nitrogen sources. Cotton was planted in 2003, 2004, and 2006. Corn was planted in 2005 as a rotation crop using a no-till planter in all plots, and no fertilizer was applied. Poultry litter application resulted in higher CO2 emission from soil compared with AN application regardless of tillage system. In 2003 and 2006, CT (4.39 and 3.40 micromol m(-2) s(-1), respectively) and MT (4.17 and 3.39 micromol m(-2) s(-1), respectively) with PL at 100 kg N ha(-1) (100 PLN) recorded significantly higher CO2 efflux compared with NT with 100 PLN (2.84 and 2.47 micromol m(-2) s(-1), respectively). Total soil C at 0- to 15-cm depth was not affected by tillage but significantly increased with PL application and winter rye cover cropping. In general, cotton produced with NT conservation tillage in conjunction with PL and winter rye cover cropping reduced CO2 emissions and sequestered more soil C compared with control treatments.     

Effect of chemical and microbial amendments on ammonia volatilization from composting poultry litter

Research has shown that aluminum sulfate (alum) and phosphoric acid greatly reduce ammonia (NH3) volatilization from poultry litter; however, no studies have yet reported the effects of these amendments on field-scale composting of poultry litter. The objectives of this study were to (i) evaluate NH3 volatilization from composting litter by measuring both NH3 volatilization and changes in total nitrogen (N) in the litter and (ii) evaluate potential methods of reducing NH3 losses from composting poultry litter. Poultry litter was composted for 68 d the first year and 92 d the second year. Eleven treatments were screened in Year 1, which included an unamended control, a microbial mixture, a microbial mixture with 5% alum incorporated into the litter, 5 and 10% alum rates either surface-applied or incorporated, and 1 and 2% phosphoric acid rates either surface-applied or incorporated. Treatments in Year 2 included an unamended control, a microbial mixture, alum (7% by fresh wt.), and phosphoric acid (1.5% by fresh wt.). Alum and phosphoric acid reduced NH3 volatilization from composting poultry litter by as much as 76 and 54%, respectively. The highest NH3 emission rates were from microbial treatments each year. Compost treated with chemical amendments retained more initial N than all other treatments. Due to the cost and N loss associated with composting poultry litter, composting is not economical from an agronomic perspective compared with the use of fresh poultry litter.     

Ammonia volatilization from marsh-pond-marsh constructed wetlands treating swine wastewater

Ammonia (NH3) volatilization is an undesirable mechanism for the removal of nitrogen (N) from wastewater treatment wetlands. To minimize the potential for NH3 volatilization, it is important to determine how wetland design affects NH3 volatilization. The objective of this research was to determine how the presence of a pond section affects NH3 volatilization from constructed wetlands treating wastewater from a confined swine operation. Wastewater was added at different N loads to six constructed wetlands of the marsh-pond-marsh design that were located in Greensboro, North Carolina, USA. A large enclosure was used to measure NH3 volatilization from the marsh and pond sections of each wetland in July and August of 2001. Ammonia volatilized from marsh and pond sections at rates ranging from 5 to 102 mg NH3-N m(-2) h(-1). Pond sections exhibited a significantly greater increase in the rate of NH3 volatilization (p < 0.0001) than did either marsh section as N load increased. At N loads greater than 15 kg ha(-1) d(-1), NH3 volatilization accounted for 23 to 36% of the N load. Furthermore, NH3 volatilization was the dominant (54-79%) N removal mechanism at N loads greater than 15 kg ha(-1) d(-1). Without the pond sections, NH3 volatilization would have been a minor contributor (less than 12%) to the N balance of these wetlands. To minimize NH3 volatilization, continuous marsh systems should be preferred over marsh-pond-marsh systems for the treatment of wastewater from confined animal operations.     

Evaluating long-term nitrogen- versus phosphorus-based nutrient management of poultry litter

Environmental concerns are driving manure management in many areas from a traditional nitrogen (N) basis toward phosphorus (P)-based nutrient management plans. We investigated how changing nutrient management from an N to a P basis affected crop yields and soil properties in high P soils over a 7-yr period. Three sites were established on farmers' fields, and at each site the same six treatments were applied for 6 or 7 yr. These treatments were (i) no P; (ii) poultry litter applied on an N basis; (iii) inorganic P, equal to the P applied in treatment 2; (iv) poultry litter applied on an estimated annual crop P removal basis; (v) inorganic P, equal to the P applied in treatment iv; and (vi) poultry litter applied once every 2 or 3 yr at a 2- or 3-yr crop removal P rate. All treatments received the same rate of plant-available N. Yields, P balance, soil pH, Mehlich 1 P, and water-soluble P (WSP) were monitored during the experiment. Over the course of the experiment, litter had the beneficial effect of raising soil pH relative to the inorganic treatments. After 7 yr, Mehlich 1 P and WSP were greatest in soils under the N-based treatments, smallest in the no P treatment, and intermediate in the P-based treatments. For example, at the Shenandoah site, Mehlich 1 P decreased by 35 mg kg(-1) under the no P treatment and increased by 36 mg kg(-1) under the inorganic N-based treatment. There were no significant differences between inorganic fertilizer and poultry litter nutrient sources. The results of this study show that soil test P can be decreased in high-P soils over a few years by changing from an N-based to a P-based nutrient management plan or stopping P applications without negatively affecting yields.     

Soil microbial communities and enzyme activities under various poultry litter application rates

The potential excessive nutrient and/or microbial loading from mismanaged land application of organic fertilizers is forcing changes in animal waste management. Currently, it is not clear to what extent different rates of poultry litter impact soil microbial communities, which control nutrient availability, organic matter quality and quantity, and soil degradation potential. From 2002 to 2004, we investigated the microbial community and several enzyme activities in a Vertisol soil (fine, smectitic, thermic, Udic Haplustert) at 0 to 15 cm as affected by different rates of poultry litter application to pasture (0, 6.7, and 13.4 Mg ha(-1)) and cultivated sites (0, 4.5, 6.7, 9.0, 11.2, and 13.4 Mg ha(-1)) in Texas, USA. No differences in soil pH (average: 7.9), total N (pasture: 2.01-3.53, cultivated: 1.09-1.98 g kg(-1) soil) or organic C (pasture average: 25-26.7, cultivated average: 13.9-16.1 g kg(-1) soil) were observed following the first four years of litter application. Microbial biomass carbon (MBC) and nitrogen (MBN) increased at litter rates greater than 6.7 Mg ha(-1) (pasture: MBC = >863, MBN = >88 mg kg(-1) soil) compared to sites with no applied litter (MBC = 722, MBN = 69 mg kg(-1) soil). Enzyme activities of C (beta-glucosidase, alpha-galactosidase, beta-glucosaminidase) or N cycling (beta-glucosaminidase) were increased at litter rates greater than 6.7 Mg ha(-1). Enzyme activities of P (alkaline phosphatase) and S (arylsulfatase) mineralization showed the same response in pasture, but they were only increased at the highest (9.0, 11.2, and 13.4 Mg ha(-1)) litter application rates in cultivated sites. According to fatty acid methyl ester (FAME) analysis, the pasture soils experienced shifts to higher bacterial populations at litter rates of 6.7 Mg ha(-1), and shifts to higher fungal populations at the highest litter application rates in cultivated sites. While rates greater than 6.7 Mg ha(-1) provided rapid enhancement of the soil microbial populations and enzymatic activities, they result in P application in excess of crop needs. Thus, studies will continue to investigate whether litter application at rates below 6.7 Mg ha(-1), previously recommended to maintain water quality, will result in similar improved soil microbial and biochemical functioning with continued annual litter application.     

Phosphorus runoff losses from subsurface-applied poultry litter on coastal plain soils

The application of poultry litter to soils is a water quality concern on the Delmarva Peninsula, as runoff contributes P to the eutrophic Chesapeake Bay. This study compared a new subsurface applicator for poultry litter with conventional surface application and tillage incorporation of litter on a Coastal Plain soil under no-till management. Monolith lysimeters (61 cm by 61 cm by 61 cm) were collected immediately after litter application and subjected to rainfall simulation (61 mm h(-1) 1 h) 15 and 42 d later. In the first rainfall event, subsurface application of litter significantly lowered total P losses in runoff (1.90 kg ha(-1)) compared with surface application (4.78 kg ha(-1)). Losses of P with subsurface application were not significantly different from disked litter or an unamended control. By the second event, total P losses did not differ significantly between surface and subsurface litter treatments but were at least twofold greater than losses from the disked and control treatments. A rising water table in the second event likely mobilized dissolved forms of P in subsurface-applied litter to the soil surface, enriching runoff water with P. Across both events, subsurface application of litter did not significantly decrease cumulative losses of P relative to surface-applied litter, whereas disking the litter into the soil did. Results confirm the short-term reduction of runoff P losses with subsurface litter application observed elsewhere but highlight the modifying effect of soil hydrology on this technology's ability to minimize P loss in runoff.     

Ammonia volatilization from surface-banded and broadcast application of liquid dairy manure on grass forage

Manure can provide valuable nutrients, especially N, for grass forage, but high NH, volatilization losses from standard surface-broadcast application limits N availability and raises environmental concerns. Eight field trials were conducted to evaluate the emission of NH, from liquid dairy manure, either surface broadcast or applied in narrow surface bands with a trailing-foot implement. Manure was applied using both techniques at rates of approximately 25 and 50 m3 ha(-1) on either orchardgrass (Dactylis glomerata L.) on a well-drained silt loam or reed canarygrass (Phalaris arundinacea L.) on a somewhat poorly drained clay soil. Ammonia emission was measured with a dynamic chamber/equilibrium concentration technique. High NH3 emission rates in broadcast treatments, especially at the high rate (2 to 13 kg ha(-1) h(-1)), occurred during the first few hours after spreading, followed by a rapid reduction to low levels (<0.5 kg ha(-1) h(-1) in most cases) by 24 h after spreading and in subsequent days. Band treatments often followed the same pattern but with initial rates substantially lower and with a less dramatic decrease over time. Total estimated NH3 losses from broadcast application, as a percent of total ammoniacal N (TAN) applied, averaged 39% (range of 20 to 59%) from the high manure rate and 25% (range of 9 to 52%) from the low rate. Band spreading reduced total NH3 losses by an average of 52 and 29% for the high and low manure rates, respectively. Results show that the trailing-foot band application method can reduce NH3 losses and conserve N for perennial forage production.     

Nitrogen oxide and methane emissions under varying tillage and fertilizer management

Comprehensive assessment of the total greenhouse gas (GHG) budget of reduced tillage agricultural systems must consider emissions of nitrous oxide (N2O) and methane (CH4), each of which have higher global warming potentials than carbon dioxide (CO2). Tillage intensity may also impact nitric oxide (NO) emissions, which can have various environmental and agronomic impacts. In 2003 and 2004, we used chambers to measure N2O, CH4, and NO fluxes from plots that had been managed under differing tillage intensity since 1991. The effect of tillage on non-CO2 GHG emissions varied, in both magnitude and direction, depending on fertilizer practices. Emissions of N2O following broadcast urea (BU) application were higher under no till (NT) and conservation tillage (CsT) compared to conventional tillage (CT). In contrast, following anhydrous ammonia (AA) injection, N2O emissions were higher under CT and CsT compared to NT. Emissions following surface urea ammonium nitrate (UAN) application did not vary with tillage. Total growing season non-CO2 GHG emissions were equivalent to CO2 emissions of 0.15 to 1.9 Mg CO2 ha(-1) yr(-1) or 0.04 to 0.53 Mg soil-C ha(-1) yr(-1). Emissions of N2O from AA-amended plots were two to four times greater than UAN- and BU-amended plots. Total NO + N2O losses in the UAN treatment were approximately 50% lower than AA and BU. This study demonstrates that N2O emissions can represent a substantial component of the total GHG budget of reduced tillage systems, and that interactions between fertilizer and tillage practices can be important in controlling non-CO2 GHG emissions.     

Effect of chemical and microbial amendment on phosphorus runoff from composted poultry litter

Environmental impacts of composting poultry litter with chemical amendments at the field scale have not been well quantified. The objectives of this study were to measure (i) P runoff and (ii) forage yield and N uptake from small plots fertilized with composted and fresh poultry litter. Two composting studies, aerated using mechanical turning, were conducted in consecutive years. Composted litter was collected at the completion of each study for use in runoff studies. Treatments in runoff studies included an unfertilized control, fresh (uncomposted) poultry litter, and litter composted with no amendment, H3PO4, alum, or a microbial mixture. An additional treatment, litter composted with alum plus the microbial mixture, was evaluated during the first year. Fertilizer treatments were applied at rates equivalent to 8.96 Mg ha(-1) and rainfall simulators were used to produce a 5 cm h(-1) storm event. Composted poultry litter, regardless of treatment, had higher total P concentrations than fresh poultry litter. Composting poultry litter resulted in reductions of N/P ratios by as much as 51%. Soluble reactive P concentrations were lowest in alum-treated compost, which reduced soluble P concentrations in runoff water by as much as 84%. Forage yields and N uptake were greatest from plots fertilized with fresh poultry litter. Composting poultry litter without the addition of C sources can increase P concentrations in the end product and surface runoff. This study also indicated that increased rates of composted poultry litter would be required to meet equivalent N rates supplied by fresh poultry litter.     

Decreasing phosphorus runoff losses from land-applied poultry litter with dietary modifications and alum addition

Phosphorus (P) losses from pastures fertilized with poultry litter contribute to the degradation of surface water quality in the United States. Dietary modification and manure amendments may reduce potential P runoff losses from pastures. In the current study, broilers were fed a normal diet, phytase diet, high available phosphorus (HAP) corn diet, or HAP corn + phytase diet. Litter treatments were untreated control and alum added at 10% by weight between flocks. Phytase and HAP corn diets reduced litter dissolved P content in poultry litter by 10 and 35%, respectively, compared with the normal diet (789 mg P kg(-1)). Alum treatment of poultry litter reduced the amount of dissolved P by 47%, while a 74% reduction was noted after alum treatment of litter from the HAP corn + phytase diet. The P concentrations in runoff water were highest from plots receiving poultry litter from the normal diet, whereas plots receiving poultry litter from phytase and HAP corn diets had reduced P concentrations. The addition of alum to the various poultry litters reduced P runoff by 52 to 69%; the greatest reduction occurred when alum was used in conjunction with HAP corn and phytase. This study demonstrates the potential added benefits of using dietary modification in conjunction with manure amendments in poultry operations. Integrators and producers should consider the use of phytase, HAP corn, and alum to reduce potential P losses associated with poultry litter application to pastures.     

Ammonia emission factors from broiler litter in barns, in storage, and after land application

We measured NH? emissions from litter in broiler houses, during storage, and after land application and conducted a mass balance of N in poultry houses. Four state-of-the-art tunnel-ventilated broiler houses in northwest Arkansas were equipped with NH? sensors, anemometers, and data loggers to continuously record NH? concentrations and ventilation for 1 yr. Gaseous fluxes of NH?, N?O, CH?, and CO? from litter were measured. Nitrogen (N) inputs and outputs were quantified. Ammonia emissions during storage and after land application were measured. Ammonia emissions during the flock averaged approximately 15.2 kg per day-house (equivalent to 28.3 g NH?per bird marketed). Emissions between flocks equaled 9.09 g NH? per bird. Hence, in-house NH? emissions were 37.5 g NH? per bird, or 14.5 g kg(-1) bird marketed (50-d-old birds). The mass balance study showed N inputs for the year to the four houses totaled 71,340 kg N, with inputs from bedding, chicks, and feed equal to 303, 602, and 70,435 kg, respectively (equivalent to 0.60, 1.19, and 139.56 g N per bird). Nitrogen outputs totaled 70,396 kg N. Annual N output from birds marketed, NH? emissions, litter or cake, mortality, and NO? emissions was 39,485, 15,571, 14,464, 635, and 241 kg N, respectively (equivalent to 78.2, 30.8, 28.7, 1.3, and 0.5 g N per bird). The percent N recovery for the N mass balance study was 98.8%. Ammonia emissions from stacked litter during a 16-d storage period were 172 g Mg(-1) litter, which is equivalent to 0.18 g NH? per bird. Ammonia losses from poultry litter broadcast to pastures were 34 kg N ha (equivalent to 15% of total N applied or 7.91 g NH? per bird). When the litter was incorporated into the pasture using a new knifing technique, NH? losses were virtually zero. The total NH? emission factor for broilers measured in this study, which includes losses in-house, during storage, and after land application, was 45.6 g NH? per bird marketed.     

Subsurface application of poultry litter and its influence on nutrient losses in runoff water from permanent pastures

Environmental pressure to reduce nutrient losses from agricultural fields has increased in recent years. To abate this nutrient loss to the environment, better management practices and new technologies need to be developed. Thus, research was conducted to evaluate if subsurface banding poultry litter (PL) would reduce nitrogen (N) and phosphorus (P) loss in surface water runoff using a four-row prototype implement. Rainfall simulations were conducted to create a 40-min runoff event in an established bermudagrass (Cynodon dactylon L.) pasture on soil types common to the Coastal Plain and Piedmont regions. The Coastal Plain soil type was a Marvyn loamy sand (fine-loamy, kaolinitic, thermic Typic Kanhapludults) and the Piedmont soil type was a Hard Labor loamy sand (fine, kaolinitic, thermic Oxyaquic Kanhapludults). Treatments consisted of surface- and subsurface-applied PL at a rate of 9 Mg ha(-1), surface broadcast-applied commercial fertilizer (CF; urea and triple superphosphate blend) at the equivalent N (330 kg N ha(-1)) and P (315 kg N ha(-1)) content of PL, and a nonfertilized control. The greatest loss for inorganic N, total N, dissolved reactive P (DRP), and total P occurred with the surface broadcast treatments, with CF contributing to the greatest loss. Nutrient losses from the subsurface banded treatment reduced N and P in surface water runoff to levels of the control. Subsurface banding of PL reduced concentrations of inorganic N 91%, total N 90%, DRP 86%, and total P 86% in runoff water compared with surface broadcasted PL. These results show that subsurface band-applied PL can greatly reduce the impact of N and P loss to the environment compared with conventional surface-applied PL and CF practices.     

Phosphorus forms and chemistry in the soil profile under long-term conservation tillage: a phosphorus-31 nuclear magnetic resonance study

In many regions, conservation tillage has replaced conventional tilling practices to reduce soil erosion, improve water conservation, and increase soil organic matter. However, tillage can have marked effects on soil properties, specifically nutrient redistribution or stratification in the soil profile. The objective of this research was to examine soil phosphorus (P) forms and concentrations in a long-term study comparing conservation tillage (direct drilling, "No Till") and conventional tillage (moldboard plowing to 20 cm depth, "Till") established on a fine sandy loam (Orthic Humo-Ferric Podzol) in Prince Edward Island, Canada. No significant differences in total carbon (C), total nitrogen (N), total P, or total organic P concentrations were detected between the tillage systems at any depth in the 0- to 60-cm depth range analyzed. However, analysis with phosphorus-31 nuclear magnetic resonance spectroscopy showed differences in P forms in the plow layer. In particular, the concentration of orthophosphate was significantly higher under No Till than Till at 5 to 10 cm, but the reverse was true at 10 to 20 cm. Mehlich 3-extractable P was also significantly higher in No Till at 5 to 10 cm and significantly higher in Till at 20 to 30 cm. This P stratification appears to be caused by a lack of mixing of applied fertilizer in No Till because the same trends were observed for pH and Mehlich 3-extractable Ca (significantly higher in the Till treatment at 20 to 30 cm), reflecting mixing of applied lime. The P saturation ratio was significantly higher under No Till at 0 to 5 cm and exceeded the recommended limits, suggesting that P stratification under No Till had increased the potential for P loss in runoff from these sites.     

Integrating local ecological knowledge and management practices of an isolated semi-arid papyrus swamp (Loboi, Kenya) into a wider conservation framework

The current study documented local utilization of Cyperus papyrus L. (papyrus), harvesting patterns, threats, and local management practices among the Endorois community living around Loboi swamp (Kenya). Papyrus is a highly productive freshwater macrophyte that is widely utilized throughout tropical Africa. However, increased human population and poverty, has led to over exploitation and conversion of papyrus wetlands to agricultural fields. Nonetheless, users of papyrus hold important local ecological knowledge (LEK) and practices. We show that Endorois practices on papyrus uses are compatible with the management priorities of the swamp and a wider conservation framework using data obtained from three focus group discussions (FGD), interviews of 34 households and 15 key informants. The study revealed that papyrus support local livelihood notably as a source of income (papyrus mats are sold), cattle fodder, roofing materials (shelter), and cooking fuel. The study further revealed important LEK relating to harvesting patterns, recovery after harvesting and traditional management practices. Correlation and principal component analyses showed that experienced old harvesters (EXPERT) avoided harvesting repeatedly at the same location (REVISIT), thereby allowing recovery of papyrus when compared to younger harvesters (r = 0.63, p < 0.01). However, over 70% of harvesters predicted a decrease in papyrus coverage in the future due to the current diversion of water from the swamp, and frequent droughts, despite the instituted traditional management strategies (e.g. rotational harvesting) to curb overharvesting. The study concluded that the documentation of site-scale papyrus users' profile, LEK, and traditional practices are vital for the conservation and management of Loboi swamp.     

Assessment of denitrification gaseous end-products in the soil profile under two water table management practices using repeated measures analysis

The denitrification process and nitrous oxide (N2O) production in the soil profile are poorly documented because most research into denitrification has concentrated on the upper soil layer (0-0.15 m). This study, undertaken during the 1999 and 2000 growing seasons, was designed to examine the effects of water table management (WTM), nitrogen (N) application rate, and depth (0.15, 0.30, and 0.45 m) on soil denitrification end-products (N2O and N2) from a corn (Zea mays L.) field. Water table management treatments were free drainage (FD) with open drains and subirrigation (SI) with a target water table depth of 0.6 m. Fertility treatments (ammonium nitrate) were 120 kg N ha(-1) (N120) and 200 kg N ha(-1) (N200). During both growing seasons greater denitrification rates were measured in SI than in FD, particularly in the surface soil (0-0.15 m) and at the intermediate (0.15-0.30 m) soil depths under N200 treatment. Greater denitrification rates under the SI treatment, however, were not accompanied with greater N2O production. The decrease in N2O production under SI was probably caused by a more complete reduction of N2O to N2, which resulted in lower N2O to (N2O + N2) ratios. Denitrification rate, N2O production and N2O to (N2O + N2) ratios were only minimally affected by N treatments, irrespective of sampling date and soil depth. Overall, half of the denitrification occurred at the 0.15- to 0.30- and 0.30- to 0.45-m soil layers, and under SI, regardless of fertility treatment level. Consequently, sampling of the 0- to 0.15-m soil layer alone may not give an accurate estimation of denitrification losses under SI practice.     

Nitrogen and phosphorus runoff losses from variable and constant intensity rainfall simulations on loamy sand under conventional and strip tillage systems

Further studies on the quality of runoff from tillage and cropping systems in the southeastern USA are needed to refine current risk assessment tools for nutrient contamination. Our objective was to quantify and compare effects of constant (Ic) and variable (Iv) rainfall intensity patterns on inorganic nitrogen (N) and phosphorus (P) losses from a Tifton loamy sand (Plinthic Kandiudult) cropped to cotton (Gossypium hirsutum L.) and managed under conventional (CT) or strip-till (ST) systems. We simulated rainfall at a constant intensity and a variable intensity pattern (57 mm h(-1)) and collected runoff continuously at 5-min intervals for 70 min. For cumulative runoff at 50 min, the Iv pattern lost significantly greater amounts (p < 0.05) of total Kjeldahl N (TKN) and P (TKP) (849 g N ha(-1) and 266 g P ha(-1) for Iv; 623 g N ha(-1) and 192 g P ha(-1) for Ic) than did the Ic pattern. However, at 70 min, no significant differences in total losses were evident for TKN or TKP from either rainfall intensity pattern. In contrast, total cumulative losses of dissolved reactive P (DRP) and NO3-N were greatest for ST-Ic, followed by ST-Iv, CT-Ic, and CT-Iv in diminishing order (69 g DRP ha(-1) and 361 g NO3-N ha(-1); 37 g DRP ha(-1) and 133 g NO3-N ha(-1); 3 g DRP ha(-1) and 58 g NO3-N ha(-1); 1 g DRP ha(-1) and 49 g NO3-N ha(-1)). Results indicate that constant-rate rainfall simulations may overestimate the amount of dissolved nutrients lost to the environment in overland flow from cropping systems in loamy sand soils. We also found that CT treatments lost significantly greater amounts of TKN and TKP than ST treatments and in contrast, ST treatments lost significantly greater amounts of DRP and NO3-N than CT treatments. These results indicate that ST systems may be losing more soluble fractions than CT systems, but only a fraction the total N (33%) and total P (11%) lost through overland flow from CT systems.     

Variable rainfall intensity and tillage effects on runoff, sediment, and carbon losses from a loamy sand under simulated rainfall

The low-carbon, intensively cropped Coastal Plain soils of Georgia are susceptible to runoff, soil loss, and drought. Reduced tillage systems offer the best management tool for sustained row crop production. Understanding runoff, sediment, and chemical losses from conventional and reduced tillage systems is expected to improve if the effect of a variable rainfall intensity storm was quantified. Our objective was to quantify and compare effects of a constant (Ic) intensity pattern and a more realistic, observed, variable (Iv) rainfall intensity pattern on runoff (R), sediment (E), and carbon losses (C) from a Tifton loamy sand cropped to conventional-till (CT) and strip-till (ST) cotton (Gossypium hirsutum L.). Four treatments were evaluated: CT-Ic, CT-Iv, ST-Ic, and ST-Iv, each replicated three times. Field plots (n=12), each 2 by 3 m, were established on each treatment. Each 6-m2 field plot received simulated rainfall at a constant (57 mm h(-1)) or variable rainfall intensity pattern for 70 min (12-run ave.=1402 mL; CV=3%). The Iv pattern represented the most frequent occurring intensity pattern for spring storms in the region. Compared with CT, ST decreased R by 2.5-fold, E by 3.5-fold, and C by 7-fold. Maximum runoff values for Iv events were 1.6-fold higher than those for Ic events and occurred 38 min earlier. Values for Etot and Ctot for Iv events were 19-36% and 1.5-fold higher than corresponding values for Ic events. Values for Emax and Cmax for Iv events were 3-fold and 4-fold higher than corresponding values for Ic events. Carbon enrichment ratios (CER) were or=1.0 for CT plots (except for first 20 min). Maximum CER for CT-Ic, CT-Iv, ST-Ic, and ST-Iv were 2.0, 2.2, 1.0, and 1.2, respectively. Transport of sediment, carbon, and agrichemicals would be better understood if variable rainfall intensity patterns derived from natural rainfall were used in rainfall simulations to evaluate their fate and transport from CT and ST systems.