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Humans have profoundly altered phosphorus (P) cycling across scales. Agriculturally driven changes (e.g., excessive P‐fertilization and manure addition), in particular, have resulted in pronounced P accumulations in soils, often known as "soil legacy P." These legacy P reserves serve as persistent and long‐term nonpoint sources, inducing downstream eutrophication and ecosystem services degradation. While there is considerable scientific and policy interest in legacy P, its fine‐scale spatial heterogeneity, underlying drivers, and scales of variance remain unclear. Here we present an extensive field sampling (150‐m interval grid) and analysis of 1438 surface soils (0–15 cm) in 2020 for two typical subtropical grassland types managed for livestock production: Intensively managed (IM) and Semi‐natural (SN) pastures. We ask the following questions: (1) What is the spatial variability, and are there hotspots of soil legacy P? (2) Does soil legacy P vary primarily within pastures, among pastures, or between pasture types? (3) How does soil legacy P relate to pasture management intensity, soil and geographic characteristics? and (4) What is the relationship between soil legacy P and aboveground plant tissue P concentration? Our results showed that three measurements of soil legacy P (total P, Mehlich‐1, and Mehlich‐3 extractable P representing labile P pools) varied substantially across the landscape. Spatial autoregressive models revealed that soil organic matter, pH, available Fe and Al, elevation, and pasture management intensity were crucial predictors for spatial patterns of soil P, although models were more reliable for predicting total P (68.9%) than labile P. Our analysis further demonstrated that total variance in soil legacy P was greater in IM than SN pastures, and intensified pasture management rescaled spatial patterns of soil legacy P. In particular, after controlling for sample size, soil P was extremely variable at small scales, with variance diminished as spatial scale increased. Our results suggest that broad pasture‐ or farm‐level best management practices may be limited and less efficient, especially for more IM pastures. Rather, management to curtail soil legacy P and mitigate P loading and losses should be implemented at fine scales designed to target spatially distinct P hotspots across the landscape. [ABSTRACT FROM AUTHOR]

There is considerable concern about ecological recovery in wetlands that have been enriched with P; however, there are few long-term studies tracking the distribution of the soil P after the termination of P loading. The Blue Cypress Marsh Conservation Area in Florida contains areas with elevated soil P levels from historical loading. The local spatial variation of soil P was determined in a 750- by 150-m area proximal to the historic surface water inflow point and a second area of the same size located in the center of the marsh with no record of historical P impacts. The average soil total P was estimated at 847 mg [kg.sup.1] in the P-enriched area and 643 mg [kg.sup.-1] in the marsh interior (unenriched). When compared with previously determined historical data, it was estimated that soil P has decreased in the P-enriched area by about 61%. Meanwhile, there was an increase of 82% in P within the unenriched area during this same time period. These results suggest that P has been mobilized from the impacted areas toward the unimpacted marsh along the water flow patterns in the wetland. These observations have implications for restoration of high-P wetland systems. Remobilization from P-impacted to unimpacted areas can expand the area of increased nutrients despite termination of P inputs into the marsh. If remobilization predominates over burial, then it is also unlikely that the overall nutrient status of the system will return to pre-impact levels within a reasonable management time frame ( doi: 10.2136/sssaj2008.0316

Anion exchange membranes are commonly used to measure readily exchangeable and microbial P in soil, yet there is little information on their interactions with organic and condensed inorganic P compounds, which can interfere with interpretation of the results. We addressed this by quantifying the sorption of a range of P compounds to a commonly used anion exchange membrane (551642S, BDH-Prolabo, VWR International, Lutterworth, UK). Sorption and recovery of orthophosphate by the membranes was complete up to 1.17 g P [m.sup.-2]. The membranes also completely recovered sodium pyrophosphate, glucose 6-phosphate, and adenosine 5'-monophosphate, as well as significant levels (Z0-60%) of 2-aminoethylphosphonic acid, sodium hexametaphosphate, and sodium phytate. Only sodium pyrophosphate, sodium hexametaphosphate, and D-glucose 6-phosphate were detected subsequently as molybdatereactive P after elution with 0.25 mol [L.sup.-1] [H.sub.2]S[O.sub.4], however, indicating their hydrolysis in the acid eluant. Solution 31p nuclear magnetic resonance spectroscopy was used to confirm the stability of the tested compounds when exposed to the membrane and the absence of significant concentrations of orthophosphate as trace contaminants ill the compound preparations. Finally, for a series of tropical wetland soils from the Republic of Panama, we found negligible difference in eluted P concentrations determined by molybdate colorimetry and inductively coupled plasma optical emission spectrometry for both unfumigated and hexanol-fumigated samples. We therefore conclude that although organic and condensed inorganic P compounds can be recovered by anion exchange membranes, this is likely to have limited impact on the analysis of soil samples. doi: 10.2136/sssaj2009.0295

The distribution of soil P among labile and nonlabile forms can be a major determinant of agriculturaI and natural ecosystem productivity. Determination of soil P pools is typically performed using operationally defined chemical fractionation methods. Most of the current fractionation techniques were developed for predominately mineral soils, thus they provide only limited information on organic P ([P.sub.o]), particularly with respect to stability. We hypothesized that the extent to which P could be extracted from organic soils, after exposure to heat, may be related to environmental recalcitrance. We investigated two thermal methods for characterizing [P.sub.o] stability in organic wetland soils, an autoclave-based and a dry heat technique. Soils from two subtropical wetlands were collected to a depth of approximately 1 m. Autoclave-extractable P was determined by subjecting soils to 128[degrees]C and 170 kPa for 90 min in an autoclave. A second set of samples was exposed to dry heat at temperatures of I60, 200, 260, 300, 360, and 550[degrees]C. The results were compared with data from a conventional chemical P fractionation scheme. Phosphorus that could be extracted using the hot water technique declined with soil depth, representing 10 to 50% of total P in surficial soils, to 5 to 10% at a depth of 60 cm. Microbial biomass P was correlated with hot water extractable P, and represented approximately 50% of the hot water extract. In the dry heat technique, increasing the extraction temperature resulted in significantly greater extraction of [P.sub.o]. The 360[degrees]C treatment was best able to distinguish between recalcitrant and labile Po' doi: 10.2136/sssaj2009.0268

Nutrient loading has been implicated as a major cause of ecological changes in Everglades wetlands. The main objective of this study was to assess changes in microbial indicators in response to nutrient loading across the Everglades landscape. Soil chemical, physical, and microbial properties were measured for nutrient-impacted and oligotrophic sites within Water Conservation Area (WCA)-1, WCA-2a, WCA-3a, and Taylor Slough of Everglades National Park. Impacts of nutrient loading were most evident by the development of gradients in floc and soil total P from peripheral to interior areas of all wetlands following the paths of surface water flow. Floc was more responsive to nutrient loading than the underlying soil. The sensitive indicators of eutrophication were floc and soil total P, microbial biomass P, and mineralized P. Total P averaged 185 and 140% greater for nutrient-impacted than oligotrophic areas for floc and soil (0-3 cm), respectively. Microbial biomass P averaged 97 and 52% higher at nutrient-impacted than oligotrophic areas for floc and soil, respectively. Mineralized P was the most sensitive indicator of eutrophication, being 689 and 135% higher at nutrient-impacted than oligotrophic areas for floc and soil, respectively. Microbial indicators in WCA-3a and Taylor Slough were more responsive to nutrient loading than in WCA-1 and WCA-2a, which received higher P loads. Delineation of nutrient-impacted and oligotrophic areas in Everglades wetlands may serve as a baseline to assess future impacts of eutrophication. The consistent pattern in the response of microbial processes to nutrient loading across the range of conditions in different Everglades wetlands demonstrates their suitability as sensitive indicators of eutrophication. Abbreviations: EAA, Everglades Agricultural Area; SOD, soil oxygen demand; WCA, Water Conservation Area.

Elevated Hg concentrations in the Everglades pose ecological and human health risks. We mapped soil total Hg concentrations per mass ([THg.sub.M]) and area ([THg.sub.A]) across the Everglades, and investigated relationships with soil properties (total P [TP] and organic matter content), community type, and hydrologic compartmentalization. Samples (n = 600) from surface soils (0-10 cm) were selected from a population of 1405 sites spanning the Everglades. Overall, 168 sites had [THg.sub.M] levels >0.2 mg [kg.sup.-1]; interpolation suggests that 23% of the Greater Everglades exceeds this threshold. Hot spots (>0.4 mg [kg.sup.-1]) were observed in eastern Water Conservation Area (WCA) 1 and west-central WCA3A; parts of WCA2A, WCA3AN, and WCA3B were locally high. Despite significant global differences in [THg.sub.M] among plant communities, differences evaluated using paired proximate sites were not significant, suggesting that large spatial scale depositional gradients govern ecosystem storage. Median [THg.sub.A] was 1.89 mg [m.sup.-2] (range 0.07-12.05 mg [m.sup.-2]), representing approximately 100 yr of atmospheric deposition at contemporary rates (~19 [micro]g [m.sup.-2] [yr.sup.-l]). Correlation between TP and [THg.sub.M] was positive in unimpacted areas (TP < 500 mg [kg.sup.-1], r = 0.69), but negative in impacted areas (TP > 500 mg [kg.sup.-1], r = -0.47), probably due to accelerated peat accretion rates in P-enriched areas. Moreover, while reverse correlation with distance from a canal for [THg.sub.M] (0.70) and TP (-0.77) supports Hg enrichment via atmospheric deposition, [THg.sub.M] hotspots in WCA3AS, WCA1, and the Holeyland and Rotenberger tracts are suggestive of local enrichment mechanisms. Finally, despite dramatic regional emissions declines, the estimated mass of Hg in surface soils across the Everglades has increased ~20% (11,000 vs. 13,100 kg) since 1996; while the statistical significance of this change is unknown, it provides a useful benchmark for future surveys. Abbreviations: BCNE Big Cypress National Preserve; BD, bulk density; EAA, Everglades Agricultural Area; ENP, Everglades National Park; HLRB, Holeyland and Rotenberger tracts; LOI, loss-on-ignition; MeHg, methyl-mercury; OK, ordinary kriging; THg, total mercury; [THg.sub.A], total mercury per area; [THg.sub.M], total mercury per mass; TP, total phosphorus; WCA, water conservation area.