Vimala D. Nair

Fate of phosphorus in Florida Spodosols contaminated with cattle manure

Abstract Phosphorus loading from dairies and beef ranches in the Lake Okeechobee watershed and the subsequent movement of the P into the drainage waters is a major factor influencing the eutrophication of Lake Okeechobee. The soils of this area are mainly Spodosols with the watertable lying between surface and spodic horizons for extended periods each year. In this study, the quantity of total P (TP) within the soil profile (A, E, Bh and Bw horizons) of dairies and beef ranches in the Lake Okeechobee basin was determined to evaluate the magnitude of P loading in these soils. The effect of cattle density was evident in TP concentrations throughout the soil profile. In the A horizon, mean TP concentrations were 1680, 165, and 34 kg P ha −1 for high, low, and nonimpacted areas, respectively. The same trend, although at lower concentrations, was evident in the E, Bh, and Bw horizons. The quantity of P considered to be potentially mobile under leaching conditions (water-soluble P, Mehlich I extractable or NH 4 Cl extractable), also followed similar trends as the TP concentrations. Based on chemical fractionation data, nearly 80% of TP in the A horizon of the highly impacted soils may be considered leachable. We calculated hat about 4000 kg P ha −1 would be available for leaching in the soil profile of the high intensity areas immediately adjacent to the dairy barns. This “labile P” appears to be solubilized slowly over a long period of time (likely several years). There seems to be no natural mechanism whereby the P is stabilized through the formation of minerals, and even if such processes do take place, a vast amount of P still remains in a form in which it is readily transported along with the drainage water. The A and E horizons had poor P retention capacities while the retention capacity of the Bh horizon varied with the soil type, Myakka ≥ Immokalee > Pomello. Due to the Low P-retention capacity in the upper horizons of these soils, there is potential for significant subsurface lateral P transport.

Carbon Storage of Different Soil-Size Fractions in Florida Silvopastoral Systems

Compared with open (treeless) pasture systems, silvopastoral agroforestry systems that integrate trees into pasture production systems are likely to enhance soil carbon (C) sequestration in deeper soil layers. To test this hypothesis, total soil C contents at six soil depths (0-5, 5-15, 15-30, 30-50, 50-75, and 75-125 cm) were determined in silvopastoral systems with slash pine (Pinus elliottii) + bahiagrass (Paspalum notatum) and an adjacent open pasture (OP) with bahiagrass at four sites, representing Spodosols and Ultisols, in Florida. Soil samples from each layer were fractionated into three classes (250-2000, 53-250, and <53 microm), and the C contents in each were determined. Averaged across four sites and all depths, the total soil organic carbon (SOC) content was higher by 33% in silvopastures near trees (SP-T) and by 28% in the alleys between tree rows (SP-A) than in adjacent open pastures. It was higher by 39% in SP-A and 20% in SP-T than in open pastures in the largest fraction size (250-2000 microm) and by 12.3 and 18.8%, respectively, in the intermediate size fraction (53-250 microm). The highest SOC increase (up to 45 kg m(-2)) in whole soil of silvopasture compared with OP was at the 75- to 125-cm depth at the Spodosol sites. The results support the hypothesis that, compared with open pastures, silvopastures contain more C in deeper soil layers under similar ecological settings, possibly as a consequence of a major input to soil organic matter from decomposition of dead tree-roots.

Agroforestry as an approach to minimizing nutrient loss from heavily fertilized soils: The Florida experience

Nutrient buildup in the soil caused by increased animal manure and fertilizer use in agricultural and forestry practices may increase the potential for their loss from the soil, leading to groundwater contamination and nonpoint source pollution. Studies in the tropics have suggested that agroforestry practices can reduce such nutrient (especially nitrogen) losses because of enhanced nutrient uptake by tree and crop roots from varying soil depths, compared to more localized and shallow rooting depths of sole crop stands. In temperate systems, such benefits have been well documented for riparian forest buffer practices. Currently, other temperate agroforestry practices are also being considered for their potential to reduce runoff and leaching of chemicals and thereby improve environmental quality within the agricultural landscape. In this regard, the ‘Florida P-Index,’ which considers both phosphorus transport characteristics and management practices, may be a useful tool in the evaluation of nutrient management practices and environmental benefits of agroforestry. Preliminary results from an alleycropping site and a silvopastoral site on two different soil types in Florida suggest that both of these agroforestry practices will likely reduce nutrient loss compared to conventional agricultural practices. The primary aspects of P-Index include consideration of transport factors such as soil erosion, soil runoff class, leaching potential, and distance from a water body along with management factors such as soil test P, P application method, and source and rate of P application. P-Index evaluation of these studies indicates that both agroforestry sites can be on a nitrogen-based nutrient management program. The relevance of some management practices such as application of manure vs. inorganic fertilizer is also discussed in light of the P-Index and the two agroforestry practices.

A capacity factor as an alternative to soil test phosphorus in phosphorus risk assessment

Abstract Soil test phosphorus (P) concentrations (STP) are often used as measures of environmental P risk. However, a low STP is not valid justification for further P application because P sorption capacity may be low and P added could be lost to surface waters. The degree of P saturation (DPS) normalises extractable P using extractable Al and Fe as a surrogate for P sorption capacity, but like STP, fails to convey a magnitude of capacity. We propose the use of a DPS‐based prediction of the remaining soil P storage capacity (SPSC) that would capture risks arising from previous loading as well as inherently low P sorption capacity. The SPSC is a direct estimate of the amount of P a soil can sorb before exceeding a threshold soil equilibrium concentration. In this paper, we demonstrate the applicability of the SPSC for a variety of sandy soils impacted by dairy and poultry manure additions. The SPSC provides a means to assess the capacity of a soil to retain additional P and hence is a more useful indicator of P‐related environmental risk than STP or DPS measures alone.

Soil carbon storage in silvopastoral systems and a treeless pasture in northwestern Spain.

Soil particle size and land management practices are known to have considerable influence on carbon (C) storage in soils, but such information is lacking for silvopastoral systems in Spain. This study quantified the amounts of soil C stored at various depths to 100 cm under silvopastoral plots of radiata pine ( D. Don) and birch ( Roth) in comparison to treeless pasture in Galicia, Spain. Soils were fractionated into three size classes (<53, 53-250, and 250-2000 μm), and C stored in them and in the whole (nonfractionated) soil was determined. Overall, the C stock to 1 m ranged from 80.9 to 176.9 Mg ha in these soils. Up to 1 m depth, 78.82% of C was found in the 0- to 25-cm soil depth, with 12.9, 4.92, and 3.36% in the 25- to 50-, 50- to 75-, and 75- to 100-cm depths, respectively. Soils under birch at 0 to 25 cm stored more C in the 250- to 2000-μm size class as compared with those under radiata pine; at that depth, pasture had more C than pine silvopasture in the smaller soil fractions (<53 and 53-250 μm). In the 75- to 100-cm depth, there was significantly more storage of C in the 250- to 2000-μm fraction in both silvopastures as compared with the pasture. The higher storage of soil C in larger fraction size in lower soil depths of silvopasture suggests that planting of trees into traditional agricultural landscapes will promote longer-term storage of C in the soil.

SOIL DEVELOPMENT IN PHOSPHATE-MINED CREATED WETLANDS OF FLORIDA, USA

Soil characteristics of a wide variety of created wetlands were compared to those of native wetlands in phosphate-mined areas from central and north Florida, USA. Criteria selected for evaluation of soil samples from 184 sites included soil compaction, bulk density, organic matter (carbon) and nitrogen content, C∶N ratio, and available and total nutrient contents. Organic matter accumulation, one of the indicators of a functional wetland, increased across transects going from uplands toward the center of the wetlands, and with wetland age. The organic matter accumulation rate in the AO and A1 horizons was 320 g m2yr−1. Native wetlands had significantly greater organic matter accumulation, both in the litter and mineral soil surface. The C∶N ratio of the soil organic matter decreased with created wetland age and approached values commonly found in wetland soils (15–25). Bulk density decreased with increasing organic matter content in the created wetlands, and low bulk density soils appeared to support better vegetative growth. Based on the above-mentioned parameters, reclaimed wetlands are slowly developing into “typical” wetlands; the rate of development could possibly be increased by minimizing soil compaction, incorporation of organic matter, or by fertilization.

Soil carbon storage in silvopasture and related land-use systems in the brazilian cerrado.

Silvopastoral management of fast-growing tree plantations is becoming popular in the Brazilian Cerrado (savanna). To understand the influence of such systems on soil carbon (C) storage, we studied C content in three aggregate size classes in six land-use systems (LUS) on Oxisols in Minas Gerais, Brazil. The systems were a native forest, a treeless pasture, 24- and 4-yr-old eucalyptus ( sp.) plantations, and 15- and 4-yr-old silvopastures of fodder grass plus animals under eucalyptus. From each system, replicated soil samples were collected from four depths (0-10, 10-20, 20-50, and 50-100 cm), fractionated into 2000- to 250-, 250- to 53-, and <53-μm size classes representing macroaggregates, microaggregates, and silt + clay, respectively, and their C contents determined. Macroaggregate was the predominant size fraction under all LUS, especially in the surface soil layers of tree-based systems. In general, C concentrations (g kg soil) in the different aggregate size fractions did not vary within the same depth. The soil organic carbon (SOC) stock (Mg C ha) to 1-m depth was highest under pasture compared with other LUS owing to its higher soil bulk density. The soils under all LUS had higher C stock compared with other reported values for managed tropical ecosystems: down to 1 m, total SOC stock values ranged from 461 Mg ha under pasture to 393 Mg ha under old eucalyptus. Considering the possibility for formation and retention of microaggregates within macroggregates in low management-intensive systems such as silvopasture, the macroaggregate dynamics in the soil seem to be a good indicator of its C storage potential.

Soil phosphorus saturation ratio for risk assessment in land use systems

The risk of phosphorus loss from agricultural soils can have serious implications for water quality. This problem has been noted particularly in sandy soils in several parts of the world including Europe (e.g., the Netherlands, Italy, and UK) and the southeastern USA. However, the capacity of a soil to retain P is limited and even non-sandy soils have the potential to eventually release P when inorganic or organic fertilizer is added over a period of time. A threshold phosphorus saturation ratio (PSR), calculated from P, Fe and Al in an oxalate or a soil test solution such as Mehlich 1 or Mehlich 3, has been recognized as a practical means of determining when a soil has reached a level of P loading that constitutes an environmental risk. When soils are below a threshold PSR value, the equilibrium P concentration (EPC0) is minimal. Further, the soil P storage capacity calculated from the same data is directly linked to the strength of P bonding (KL) as determined from Langmuir isotherms, and KD, the distribution coefficient related to the strength of sorption. While the PSR is occasionally used as a predictor of the onset of environmentally significant P loss from a soil, the procedure might be adopted as a routine soil test.

Nitrogen mineralization in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

Information on temporal and spatial patterns of N mineralization is critical in designing tree-crop mixed systems that could maximize N uptake while minimizing N loss. We quantified N mineralization rates in a pecan (Carya illinoensis K. Koch)–cotton (Gossypium hirsutum L.) alley cropping system in northwestern Florida with (non-barrier) and without tree-crop belowground interactions (barrier separating the root systems of pecan and cotton). Monthly rates of mineralization were estimated using buried bag incubations over a 15-month period. In addition, seasonal mineralization rates and cotton lint yield on soils supplied with two sources of N—inorganic fertilizer and organic poultry litter—were assessed. Results indicated that temporal variations in net NH4 and NO3 accumulation and mineralization rates were driven primarily by environmental factors and to a lesser degree by initial soil NH4 and NO3 levels. Mineralization varied by belowground interaction treatment during the initial growing season, when the non-barrier treatment exhibited a higher mineralization rate than the barrier treatment, likely due to reduced nutrient uptake by cotton in the non-barrier or a higher degree of immobilization in the barrier treatment. Mineralization during the second growing season was similar for both treatments. Source of N had no effects on N transformation in the soil. Lint yield reductions were observed in the non-barrier treatment during both years compared to the barrier treatment, likely due to interspecific competition for water. Yield differences between treatments in the second growing season were likely compounded by a diminishing pre-study fallow effect. Source of N was found to have a significant effect on cotton yield, with inorganic fertilizer resulting in 39% higher lint compared to poultry litter in the barrier treatment.

Development of indices to predict phosphorus release from wetland soils.

The U.S. Environmental Protection Agency created the Clean Water Action Plan to develop nutrient criteria for four water body types: lakes and reservoirs, rivers and streams, estuaries, and wetlands. Significant progress has been made in open water systems. However, only areas in and around the Florida Everglades have had numeric nutrient criteria set, due to the complexity, heterogeneity, and limited information available for wetlands. Our objective was to evaluate various soil tests to predict significant P release potential of soil in wetlands. A total of 630 surface soil samples (0-10 cm) were collected for this study from four southeastern states: Florida, Alabama, Georgia, and South Carolina. Soil samples were collected from the center of wetlands, the edge of the wetlands, and from adjacent uplands. The phosphorus saturation ratios (PSR), calculated using P, Fe, and Al molar concentrations from Mehlich 1 (M1-PSR), Mehlich 3 (M3-PSR), and oxalate (Ox-PSR) extractions and the amount of P extracted by different extractants were used to predict P loss potential from a soil. Total phosphorus (TP) concentration in wetland soils, estimated as the 75th percentile of the distribution of least impacted wetland soils as an example, was approximately 550 mg kg(-1). Based on this reference background condition, procedures for obtaining threshold values for P release to the surrounding water bodies were developed and threshold values calculated: M1-P = 24 mg kg(-1), M3-P = 44 mg kg(-1), Ox-PSR = 0.079, M1-PSR = 0.101, and M3-PSR = 0.067.