Labile soil organic carbon and nitrogen within a gradient of dryland agricultural land-use intensity in Wyoming, USA

Tillage in dryland winter wheat (Triticum aestivum L.)-summer fallow cropping systems of the central High Plains, USA, has caused significant erosion and loss of soil organic matter (SOM), underscoring the need for more sustainable practices on marginally-productive semiarid lands. Conversion to no-till (NT) wheat-fallow or perennial grass-legume cover, as in the Conservation Reserve Program (CRP), provides viable alternatives to tillage-based wheat-fallow practices. However, the overall impact of transitioning on soil biogeochemistry is not fully understood. Labile SOM pools were determined on soil cores (0-120 cm) collected in July 2011 from five unfertilized fields that spanned a gradient in disturbance associated with land-use practices: a more intensively tilled wheat-fallow ('historic', HT) than a conventional (CT) wheat-fallow with limited herbicides combined with tillage, NT wheat-fallow that exclusively used herbicides for ≥ 10 years, formerly cultivated HT wheat-fallow planted to grass-legume mixture as in the CRP for 7 years, and native prairie (NP) with no history of agricultural disturbance. Significant differences in microbial biomass C (MBC), potentially mineralizable C (PMC) and nitrogen (PMN) were largely confined to the surface soil (0-30 cm), following the order NP > CRP > all wheat-fallow systems. When expressed on a per soil organic C (SOC) basis, both MBC and PMC followed the order CRP > NP > all wheat-fallow systems. In contrast, when normalized by total soil N (TN), PMN was higher in HT and CT soils than in other soils (NT, CRP, and NP). MBC and PMC were positively correlated with SOC whereas PMN, soil pH, and electrical conductivity (EC) showed a negative relationship with SOC. These results suggest more efficient conservation of SOM under perennial grass-legume system than NT wheat-fallow rotation. We suggest that cessation of tillage alone may not be sufficient for the recovery of labile-pool SOM degraded through long-term cultivation in the absence of inputs for soil fertility renewal.