Intensification of dairy production increases cow numbers and increases nutrient loading on associated land resulting in non-point source pollution in surrounding watersheds. Diversifying cropping systems and employing alternative manure management strategies could improve environmental quality and contribute to sustainable dairy production. The objectives were to evaluate cropping systems that enable sustainable nutrient management and improved environmental quality for dairy production systems in the southeastern US. Nutrient imports, exports, and forms in soil were compared between Tifton 85 bermudagrass grown for forage and Tifway bermudagrass grown for turfgrass sod under field conditions and varied composted manure (CM) management practices. In addition, a mineralization study was conducted under laboratory conditions to evaluate responses of soil biological and chemical processes to CM nutrients and to relate those processes to nutrient export and environmental quality. Furthermore, relationships of CM management to runoff losses of nutrients and carbon were evaluated under simulated rainfall for each the forage and turfgrass production systems. One harvest of Tifway sod removed 4-fold greater amounts of CM sources of total N and total P than three harvest of Tifton 85 forage biomass. Treating CM with Alum did not affect the amount of total N or P exported in Tifway sod or Tifton 85 forage. Amending soil with CM did result in immobilization of N and reduced biomass production for Tifway and Tifton 85 compared to control soils. Yet, treating CM with Alum before incorporating or topdressing soil reduced the concentration of water extractable P (WEP) in CM 95.1%. Similar to CM, Alum reduced the WEP concentration of amended soil 89 to 91%. The affect of Alum on reducing WEP in CM and CM amended soil was also observed in runoff water. Treating CM with Alum reduced runoff concentration of TDP to levels comparable to control soil over both rain events. The diversification of crop production systems to include turfgrass sod and use of alternative CM management practices will enable producers to optimize and improve net export of nutrients from dairies while protecting water quality.
Similar to other regions in the US, the intensification of dairy production within Texas and other southeastern states has contributed to localized increases in cow numbers, nutrient loading on associated land, and concerns about off-farm environmental impact. Manure and wastewater supply nitrogen (N) for cropping systems on dairies and surrounding watersheds, but 20% or less of land applied phosphorus (P) is typically removed in forage harvests (Sanderson and Jones, 1997). Excess P remaining in soil is a potential nonpoint source of pollution and Texas regulations now mandate export of manure nutrients from dairies and specified watersheds to protect rivers and lakes. Similar situations exist across the USA (Beegle and Lanyon, 1994).
In an effort to prevent pollution from land-applied manure and wastewater, uptake of N and P in forage harvests has been evaluated for year-round cropping systems in the Southern Region. Adapted forage crops are grown and harvested to remove P and prevent nonpoint-source losses of N and P. Woodward et al. (2007) reported annual harvests from a bermudagrass/rye cropping system removed 67 kg P ha-1 cycle-1. Ketterings et al. (2006) reported a two-cut system applied to brown midrib sorghum removed up to 510 kg N ha-1 yr-1 and 101 kg P ha-1 yr-1. Brink et al. (2004) observed average uptake and export of 300 kg N ha-1 yr-1 and 46.5 kg P ha-1 yr-1 for Tifton 85 bermudagrass.
A previous report identified an option for export of two to three times more total P than the year-round forage harvests. A single harvest of Tifway bermudagrass sod exported up to 561 kg ha-1 of total N and 219 kg ha-1 of total P applied as raw or composted dairy manure (Vietor et al. 2002). In addition, an economic analysis indicated turfgrass sod sales were sufficient to purchase forage to replace that grown on the land area allocated to sod production (Vietor et al. 2003).
The introduction of turfgrass sod will diversify forage systems designed to achieve the dual purposes of cow nutrition and nutrient management. Yet, crop and soil responses and export of applied nutrients remain to be compared between forage and sod crops within the same space and time. The fate of manure applied nutrients in soil, which includes sorption reactions, mineralization of N, P and C, and uptake of nutrients by plants, will impact export and nonpoint-source losses (Jiao et al. 2007, Oehl et al. 2001). A side-by-side comparison between sod and forage production is needed to evaluate soil and crop responses and nutrient imports and exports for waste application fields and dairies. In addition, suitable waste management practices are needed for each crop to ensure the sustainability of dairy production in impaired watersheds.
The comparison between forage and turfgrass sod production systems will reveal advantages and disadvantages of diversifying crop enterprises used to manage nutrients in dairy production systems. In addition, measurements of soil, plant, and water quality responses will reveal the comparative effects of forage and turfgrass production practices on components of sustainability for dairy production systems. For example, amending composted manure (CM) with Alum can significantly reduce water extractable P in CM and soluble reactive P in runoff water, which could enhance the feasibility of topdressing CM after sod or forage harvests (DeLaune et al. 2006). Conversely, incorporation of CM may be necessary to protect environmental quality. Incorporation of CM will increase P adsorption to soil particles and limit nonpoint source losses in runoff. In addition, incorporation of large CM rates could improve soil water retention and biomass production compared to surface application (Kleinman et al. 2002). Knowledge of the integrated effects of forage or turfgrass production systems and associated practices on soil processes and plants will enable dairy producers to optimize nutrient management and land allocation to forage and turf crops.
- Compare forage and turfgrass sod production systems with respect to 1.) field-scale nutrient imports and exports and 2.) responses of plant and soil biological and chemical processes to manure management practices.
Compare runoff losses of N, P, and organic C among manure management practices for forage and turfgrass sod production systems.
Compare mass balance of N and P on field and dairy scale between manure management practices and forage and turfgrass sod production systems.
Soil responses to topdressed and incorporated CM, with and without treatment using commercially available Alum (1.0 g Al kg-1 Alum) to manage water soluble P concentrations, was compared between Tifton 85 bermudagrass forage and Tifway bermudagrass turf. Four replications of the eight combinations of crop, application method, and Alum treatment, plus controls for each crop, comprised a randomized block design. The CM used was a compost derived from the byproduct of anaerobic digestion of dairy manure. The CM with and without Alum was incorporated to a 5-cm depth of a Boonville fine sandy loam soil to provide 500 kg ha-1 TP before planting of forage and turfgrass in May of 2008. For topdressed treatments, 250 kg ha-1 of TP was applied as CM after planting. Alum was mixed with CM to achieve a ratio of 0.1 kg Alum per kg of dry CM prior to application to designated treatments (Vietor et al. 2009).
Digital photography and ImageJ software was used to measure and analyze leaf growth and soil coverage rates monthly during establishment of forage and turf crops. Tifway turf was mowed after reaching a height of 7.5-cm to a 2.5-cm height and clippings weighed, sub-sampled for analysis, and returned. Tifton 85 forage was cut to a 10-cm height, weighed, sub-sampled for analysis, and removed from plots every 28 days or as growth permitted following the initial harvest in 2008.
Soil and CM were sampled and analyzed before CM applications were made in April of 2008. Soil was sampled from a 0- to 5-cm depth and 5- to 20-cm depth in May of 2008. During harvest of Tifway sod in October of 2008 and 2009, sod was cut (45 cm width, 2 cm depth) using a mechanical sod harvester. Sod was sampled by removing four 10 cm diameter cores from cut sod. In addition, soil was sampled from a 2- to 5-cm and 5- to 20-cm depth below the cut sod. For Tifton 85 plots, soil was sampled from 0-5 cm and 5-20 cm depths following the final forage harvest in 2008 and 2009. Total N, P, cations, organic C; Mehlich-3 and water extractable P; and KCl-extractable NO3-N and NH4-N were analyzed using appropriate digest and extractions. Turfgrass separated from soil at sod harvest and forage harvests was sampled and analyzed to quantify total N, P and minerals taken up and exported in above ground biomass.
A mineralization study was conducted over a 28-day period for soil sampled from the 10 treatments three weeks after topdressing or incorporation of CM. Intact soil cores (6-cm diameter, 5-cm depth) were incubated at 25° C for 28 days in 1-L jars with an alkali CO2 trap containing 10 ml 1.0 N NaOH. Traps were changed at 1, 3, 7, 14, 21 and 28 days and titrated with 1.0 N HCl to measure carbon mineralization. Nitrogen mineralization was measured (colorimetrically) as the differences in inorganic N (NO3-N and NH4-N) between samplings before and after the incubation over 28 days. The mineralization rates were used to estimate monthly application rates of fertilizer N for the turfgrass and forage bermudagrass amended with CM.
Three replications of the four field treatments comprising topdressed or incorporated CM with or without Alum, plus a control, was installed on a Boonville fine sandy loam soil for each Tifway turf and Tifton 85 forage in box lysimeters (74 x 34 x 15 cm). Runoff concentrations and losses of total and dissolved N, P and organic C were measured under simulated rain (10 cm hr-1) during establishment and after complete forage or turfgrass coverage of the soil surface. The lysimeters were maintained under greenhouse conditions before and between the simulated rain applications.
Runoff volumes were measured and sampled at 10-min intervals over a 30-min period after runoff starts. Samples were refrigerated and filtered for analysis of NO3-N, NH4-N, and dissolved reactive P (DRP) within 24 hr. Total N and P and total dissolved P and organic C was analyzed within 3 d after sampling. Soil was sampled from 0- to 5-cm and 5- to 15-cm depths for analysis of total N, P, and organic C after the second runoff sampling. On the same date, grass was clipped to the soil surface, dried, weighed, and sampled. Turfgrass and forage was sampled and analyzed using Kjeldahl digestion procedures. The CM and soil samples were analyzed following Kjeldahl digestion or extraction with Mehlich-3 or deionized water.
The General Linear Model procedure of SPSS 15.0 was used for Analysis of Variance among treatments and sampling dates. Fisher’s least significant difference (LSD) was used to separate means. Regression analysis was used to relate runoff losses and export of nutrients and organic C to concentrations and mass of nutrients and organic C in soil and applied CM.
The plot-scale measurements of N and P imports, exports, and soil storage during establishment, production, and harvest was used to estimate nutrient balance ha-1.
Prior to imposing soil treatments and planting of sprigs, soil and CM was sampled and analyzed to determine chemical characteristics. The total P (TP) concentration of CM with and without Alum was used to calculate appropriate application rate of CM. An average of 41.5 Mg ha-1 of dry CM with or without Alum was topdressed on Tifway and Tifton 85 plots in May of 2008 to achieve the target TP rate of 250 kg TP ha-1. For soils treatments that included incorporated CM, an average of 81.4 Mg ha-1 of dry CM was applied and incorporated to a 5 cm depth to achieve a TP application rate of 500 kg TP ha-1 in May of 2008. In October of 2008, an additional surface application (40.7 Mg ha-1) of dry CM was applied following sod removal or final forage harvest. For CM with Alum, Alum was mixed with water (1:2; Alum to water ratio) to dissolve and applied to CM (0.1 kg of Alum per kg of dry CM) and mixed in a portable cement mixer for 5 min before application to soil. Analysis of CM indicated that Alum addition reduced (P = 0.05) water extractable P (WEP) concentration 95.1% (1:200 CM to water extraction ratio) compared to CM without Alum.
Following planting, soil was sampled and analyzed to determine the impact of CM application method and treatment of CM with Alum on soil chemical and physical properties. Crop species had no impact on soil chemical properties immediately after planting. Incorporating CM, with and without Alum, reduced (p =0.05) soil bulk density compared to control soil and soil with topdressed CM with and without Alum. It was also observed that treating CM with Alum prior to land application reduced soil pH compared to control soil in the 0- to 5-cm depth of soil for topdressed and incorporated CM. In addition, application of CM with or without Alum to soil increased soil concentration of total N (TN), total K (TK) and organic carbon (SOC) compared to control soil. However, topdressing CM with and without Alum or incorporating CM with Alum did not increase soil concentration of TP, yet did increase Mehlich-3 extractable P (M3P) compared to control soil. In contrast, CM applications increased soil TK but did not increase Mehlich-3 extractable K for all soils receiving CM compared to control soil except for soils where CM without Alum was incorporated. Similarly, increases in soil TN after CM application did not result in increases in soil NO3-N compared to control soil. The changes in concentration of SOC, TN and TK following CM application with little affect on extractable forms of N and K suggest that N and minerals found in CM were primarily in organic form. Topdressing 250 kg ha-1 or incorporating 500 kg ha-1 of CM sources of TP did not elevate soil TP concentration as expected yet did increase extractable forms of P in soil.
In addition to analyzing soil chemical properties, intact soil cores (6-cm diameter, 5-cm depth) were collected three weeks after treatments were imposed and incubated for 28 days to estimate potential C and N mineralization and to determine inorganic N fertilizer application rates. Similar to soil chemical properties, crop species did not have a significant effect on carbon mineralization. After one week of incubation, carbon mineralization had reached a constant rate and significant differences in carbon mineralization rates were observed between soil treatments. Topdressing CM with or without Alum resulted in carbon mineralization rates three times greater than control soils. In addition, incorporating CM with or without Alum resulted in greater than a five-fold increase in carbon mineralization rates compared to control soil. Although Alum did affect soil chemical properties, it did not affect decomposition rates of CM. Similarly, application rate and method did not affect decay rates. Differences in C respiration rates were largely due to the rate of CM application. An average of 12.5 Mg ha-1 of C was applied with topdressed CM and 24.4 Mg ha-1 of C was applied with incorporated CM with and without Alum. Subtracting cumulative carbon evolved from control soil from soil amended with CM, the mass of CM sources of carbon remaining after 14 and 28 days of incubation was calculated. The mass loss of carbon from 14 to 28 days was fitted to the model Lt = L0e-kt and used to estimate decay rates. Where Lt = CM-C mass at time t, L0 = CM-C mass at time 0, t = time, k = decomposition rate. The mean k-values or decomposition rates for topdressed or incorporated CM with and without Alum were similar (p > 0.05), from 0.1744 to 0.2629. An estimated 77 to 81% of the original 12.5 Mg C ha-1 applied with topdressed CM with and without Alum would remain after one year. For incorporated CM with and without Alum, about 83 to 84% of CM applied C would remain after one year of the 24.4 Mg ha-1 that was applied.
To estimate potential N mineralization, inorganic N (NH4-N + NO3-N) was measured pre- and post- incubation period. Mineralized N was calculated as the difference in inorganic N from the start of incubation through 28 days. After 28 days of incubation, no significant difference was observed in the amount of inorganic N mineralized between soils amended with CM, with or without Alum, compared to control soil. Over 28 days of incubation, 1.96 to 16.78 kg ha-1 of inorganic N was mineralized within the 0 to 5 cm depth of soil. High carbon respiration rates of CM amended soil compared to control soil with no increase in N mineralization suggest that immobilization of N may be occurring. Estimated N mineralization rates did not warrant reductions in inorganic fertilizer rates to achieve forage and turfgrass sod production. In contrast, previous studies evaluating manure and compost amended soils determined that N mineralization rates were sufficient to warrant partial reductions in N application rates (Flavel and Murphey, 2006). For Tifway Plots, 200 kg N ha-1 year-1 of ammonium sulfate (21-0-0) was applied and 400 kg N ha-1 year-1 of ammonium sulfate was applied to Tifton 85 plots.
In addition to measuring available soil nutrients and mineralization of CM sources of nutrients, biomass (dry matter) production was measured to compare soil treatments. Biomass was harvested for Tifway and Tifton 85 in July, August and September of 2008. Total dry matter production was greater (p = 0.05) for Tifton 85 compared to Tifway during 2008. In addition, differences in dry matter production were observed between soil treatments within Tifway and Tifton 85 plots. For Tifway, topdressing CM with or without Alum reduced (p = 0.05) dry matter production compared to incorporated CM without Alum and control soil. In contrast, topdressing or incorporating CM with or without Alum reduced (p = 0.05) dry matter production compared to control soil for Tifton 85. As observed during soil incubation, immobilization of N may have limited dry matter production for Tifway and Tifton 85 grown on soil amended with CM compared to control soil.
To evaluate water quality for soil treatments imposed under field conditions, box lysimeters were packed with soil and amended by topdressing or incorporating CM with and without Alum. Similar to field plots, topdressing or incorporating CM with Alum reduce (p = 0.05) soil pH compared to control soil and CM amended soil without Alum. In addition, amending soil with CM, with or without Alum, increased concentration of soil organic carbon, total P and Mehlich-3 extractable P compared to control soil. However, treating CM with Alum before incorporating or topdressing on soil reduced concentration of WEP to levels similar to control soil. Treating CM with Alum before incorporating or topdressing on soil reduced WEP concentration of soil 88.8 to 90.1% compared to soil amended with CM without Alum.
Analysis of TDP concentration in runoff water collected over two rainfall events for Tifway and Tifton 85 revealed variations in CM concentration of WEP and application method. Tifway and Tifton 85 were combined to analyze the affect of soil treatments. Differences in rain events were observed and were analyzed separately. Treating CM with Alum before topdressing or incorporating reduced (p = 0.05) concentration of TDP in runoff water compared to CM without Alum on the first and second rain event. Amending soil with Alum treated CM reduced the concentration of TDP in runoff water to levels comparable to control soil for both rain events. In addition, incorporating CM without Alum reduced the concentration of TDP in runoff water compared to topdressed CM without Alum.
Variation in concentration of WEP of CM and application rate of topdressed and incorporated CM contributed to variation in concentration of soil WEP. The rate of incorporated CM was 1.96 fold greater than the rate of topdressed CM. Similarly, concentration of soil WEP was 2.06 fold greater for incorporated CM without Alum compared to topdressed CM without Alum. Yet, the concentration of TDP in runoff water from soil amended by incorporating CM without Alum was reduced 50.4 to 80.4% compared to soil receiving topdressed CM without Alum. Regression analysis indicated that a linear relationship (r2 = 0.921) exist between soil WEP and concentration of TDP in runoff during the first rain event for control soil and soil with incorporated CM, with and without Alum. Similarly, a linear relationship (r2 = 0.575) was observed between soil WEP and mean concentration of TDP in runoff during the first rain event for control soil and soil topdressed with CM, with and without Alum. In addition, the slope was significantly (p = 0.05) different between soils with topdressed and incorporated CM with and without Alum. For contrasting methods of CM application, relationships used to predict runoff loss of TDP should be developed separately (Vietor et al., 2004). The greater slope associated with topdressed CM compared to incorporated CM indicates that incorporating CM reduces interaction of CM sources of P with rainfall and runoff and can reduce runoff loss of TDP.
For evaluating the percentage of applied nutrients exported in sod, previous studies have subtracted the amount of nutrients in control soil sod from compost amended sod to account for antecedent soil nutrients (Schnell et al., 2009). In the current study, variation of sod soil weights between control sod and CM amended sod would result in an over estimation of antecedent soil nutrients. To account for antecedent soil nutrients, the sod soil weight from CM amended soil was divided by the sod soil weight of control soil and antecedent soil nutrients were calculated by multiplying the proportion of sod soil weight by the mass of nutrients found in control sod. The mass of antecedent soil nutrients was subtracted from sod grown on CM amended soil to calculate the percentage of CM applied nutrients removed by one sod harvest. To calculate the percentage of applied nutrients removed by three forage harvests, the mass of nutrients contained in Tifton 85 dry matter was divided by the total amount of TN and TP applied.
Variations in soil chemical properties measured for Tifway and Tifton 85 for soil amended with and without CM, with and without Alum, also contributed to variations in the amount of nutrients exported from Tifton 85 forage and Tifway sod harvest. One harvest of Tifway sod removed significantly (p = 0.05) more TN and TP than three forage harvests from Tifton 85. Topdressing or incorporating CM in soil, with and without Alum, increased export of TN and TP for Tifway sod compared to control sod. Additionally, treating CM with Alum did not affect export of TN or TP in Tifway sod and Tifton 85 dry matter. One harvest of Tifway sod removed 4.04-fold greater mass of TN and 4.14-fold greater mass of TP than three harvests of Tifton 85 forage. The greatest proportion of TN and TP exported in Tifway sod was found within the soil fraction. About 75 to 83% of TN mass in sod was in the soil fraction and 81 to 87% of TP mass in sod was in the soil fraction. This demonstrates that removing a thin layer of soil (2-cm depth) during sod harvest increases the potential for nutrient export compared to removing crop biomass alone.
Although Tifton 85 dry matter production was greater than Tifway dry matter production, cycling Tifway clippings back to the soil surface throughout the growing season may also contribute to increasing soil TN and TP exported by sod. Uptake of nutrients by Tifway from below the depth of sod harvest could contribute to nutrient accumulation near the surface as clippings are harvested and distributed on the soil surface. Tifway clippings returned to the soil surface contributed from 83 to 150 kg TN ha-1 and 16 to 28 kg TP ha-1 prior to the first sod harvest. The mass of TN returned to the soil surface as clippings was from 9 to 28% of the mass of TN exported in sod and the mass of TP returned to the soil surface as clippings was from 7 to 28% of the mass of TP exported in sod.
In contrast to the total amount of TN and TP exported for Tifway sod and Tifton 85 dry matter, topdressing or incorporating CM did affect the percentage of CM applied TN and TP that was exported. Topdressing CM, with and without Alum, on soil for Tifway sod production resulted in a greater proportion of CM applied TN being exported than was exported from soil with incorporated CM, with and without Alum. Topdressing or incorporating CM, with and without Alum, did not affect the percent of CM applied TN that was exported by Tifton 85 dry matter. Similar to TN export, a greater proportion of topdressed CM applied TP was exported by Tifway sod and Tifton 85 dry matter compared to incorporated CM, with and without Alum. Applying double the rate of CM and incorporating in soil below the depth of a single sod harvest reduced the export of CM applied TN and TP compared to topdressed CM. In addition, the depth of a single sod harvest should exceed the depth of topdressed applied CM, yet export percentages of topdressed applied CM nutrients was below 100%. This suggests that mobile N and P species may have infiltrated below the depth of sod harvest or were transported in surface runoff water.
In addition, topdressing or incorporating CM affected export percentages of TP for Tifton 85. The increase in percent export of TP by Tifton 85 dry matter for topdressed compared to incorporated CM suggest that plant uptake of topdressed CM sources of TP was greater than for incorporated CM. In contrast to topdressed CM, incorporating CM, with and without Alum, may have increased interaction of P with soil and reduced plant uptake. The interaction of CM sources of P with soil was also observed during the runoff experiment, with reduced loss of TDP in runoff for incorporated CM compared to topdressed CM.
Diversifying crop production systems to include Tifton 85 forage and Tifway sod and use of alternative CM management practices can affect soil chemical, physical and biological properties and improve environmental quality. Treating CM with Alum reduced the concentration of WEP in CM and CM amended soils. Moreover, treating CM with Alum prior to land application reduced the concentration of TDP in runoff water from CM amended soils. In addition, treating CM with Alum did not affect crop biomass production or nutrient export in Tifway sod or Tifton 85 compared to CM without alum. Yet, mineralization of CM sources of TN may not be sufficient to reduce application rates of fertilizer N for Tifway and Tifton 85 production. One harvest of Tifway sod exported four-fold greater amounts of CM applied TN and TP compared to three harvest of Tifton 85 biomass. To maximize export of CM applied nutrients, crop production systems should be diversified to include turfgrass sod. Diverse cropping systems and proper management of CM and nutrients will lead to environmentally responsible and sustainable dairy production in the southeastern US.