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Sequential fractionation coupled with phosphatase hydrolysis allows a greater understanding of the effects of animal manure on the chemical distribution of soil P. Concentrations of specific soil P fractions were determined after long-term (>10 yr) poultry litter application at rates of 4.5, 6.7, 9.0, 11.2, and 13.4 Mg manure ha-1 yr-1 to watershed-scale plots (cultivated and grazed-ungrazed pasture) on a calcareous Texas Blackland Vertisol. Soil total extractable P (Pt) and inorganic P (Pi) were quantified following sequential extraction with H2O, NaHCO3, NaOH, and HCl. Hydrolyzable organic P (Pe) and non-hydrolyzable organic P (Pne) were determined in the extracted fractions following enzymatic hydrolysis. Litter application increased Pt regardless of land-use type compared with the control. Concentrations of H2O-extractable Pi in litter-amended plots increased by 9 to 34% (cultivated) and 7 to 30% (pasture) over the control, indicating substantial risk of soluble P runoff. Labile organic P (Po) extracted with H3O and NaHCO3 decreased in the order monoester > nucleic acid > phytate > Pne. An average of 68% of Pt was extractable with HCl. Organic P comprised the majority (95%) of HCl-Pt; however, only trace levels of HCl-Po were hydrolyzable, and litter application increased HCl-Pne up to 217%. Thus, litter application increased levels of both soluble Pi and stable Pne, but the specific response varied with application rate and land management. This study increased understanding of P chemical distribution with time in litter-amended soil with high clay and CaCO3 contents under differing land-use scenarios. [ABSTRACT FROM AUTHOR]

Water extractable organic matter (WEOM) is critical for soil fertility because of its role as a readily available C substrate for microbial metabolism. In the semi‐arid Texas High Plains, dryland cropping systems are common because of limited precipitation and irrigation water. Little is known about the long‐term effects of dryland cropping on soil WEOM content and composition, and how these differ from native grassland. The objective of this study was to use soils collected near Amarillo, TX, in 1977 and 2013 to examine the long‐term (72 yr total cropping length) effects of dryland cropping systems [continuous wheat and wheat–fallow rotations] and tillage methods (disk plow, stubble‐mulch tillage, and delayed stubble‐mulch tillage) on WEOM characteristics, compared with native grassland. In cropped plots, water‐extractable organic carbon (WEOC) concentrations, a measure of WEOM content, were depleted regardless of tillage or crop, with reductions up to 13% in 1977 and 60% in 2013, compared with native grassland in 2013. Other parameters of WEOM changed over time, where cropped soils from 2013 had proportionally more WEOM in aromatic, phenolic, and lignin‐like forms, attributed to recently incorporated crop residues. In contrast, WEOM in cropped soils from 1977 and native grassland (2013) contained more functional groups (e.g., aliphatic, aromatic, polysaccharide, phenol, etc.) and humic‐like substances than cropped soils in 2013. This suggests that long‐term dryland cropping led to depletion of stable, humic‐like WEOM. Enhancement of WEOM stocks and long‐term sustainability in the Texas High Plains may require more vigorous conservation systems than currently used. Core Ideas: Long‐term dryland cropping systems depleted soil dissolved organic C concentrations by up to 60% over time.Water‐extractable organic matter in cropped soils was in simple aromatic forms, while grassland dissolved organic C was more humic‐like.Long‐term dryland cropping led to depletion of stable, humic‐like water‐extractable organic matter.Vigorous conservation systems should be adopted to improve water‐extractable organic matter stocks.Soil C sequestration may be difficult in regions where limited water constrain C inputs. [ABSTRACT FROM AUTHOR]