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Title: Land conversion to irrigated agriculture reduces labile and increases recalcitrant carbon in a semi-arid Nevada soil

Author
item TRIMBLE, BRITTANY - University Of Nebraska
item Calderon, Francisco
item POULSON, SIMON - University Of Nebraska
item VERBURG, PAUL - University Of Nebraska

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2018
Publication Date: 6/22/2018
Citation: Trimble, B.R., Calderon, F.J., Poulson, S., Verburg, P.S. 2018. Land conversion to irrigated agriculture reduces labile and increases recalcitrant carbon in a semi-arid Nevada soil. Soil Biology and Biochemistry. 2:3-38. https://doi.org/10.3390/soilsystems2030038.
DOI: https://doi.org/10.3390/soilsystems2030038

Interpretive Summary: In this field based research, we compared the soil organic matter in irrigated alfalfa and naïve rangeland in Nevada. We show the negative impact of irrigation and cultivation on soil carbon stocks. We also used laboratory incubations and spectroscopic analyses to show that the soil C in cultivated soils is more chemically resistant to decomposition compared to shrubland soils.

Technical Abstract: Throughout previous decades, land-use change has contributed to rising atmospheric carbon dioxide (CO2) concentrations by reducing carbon (C) storage and increasing C emissions from previously natural ecosystems. Due to the scarcity of arable land, semi-arid rangelands are often converted to irrigated croplands, which is likely to have a large effect on soil organic carbon (SOC) due to changes in C inputs to the soil as well as environmental factors regulating decomposition. The objective of this study was to quantify the long-term effects of converting an unmanaged semi-arid shrubland into irrigated agricultural land on SOC dynamics. We compared a native rangeland dominated by rubber rabbitbrush (Ericameria nauseosa) and yellow rabbitbrush (Chrysothamnus viscidiflorus) with an adjacent alfalfa (Medicago sativa) field that has been under irrigated agriculture for at least five decades near Reno, NV. Carbon and nitrogen (N) analysis of particle size and density fractions revealed that irrigation and management significantly reduced the amount of C and N in the soil. The amount of C in the labile fractions from both the particle size fractionation and density fractionation was significantly smaller and the relative amount of C in recalcitrant fractions increased in the alfalfa field. The differences in d13C values of the soil organic matter reflected differences between dominant vegetation types, but these differences were only significant for density fractions. Both fractionation methods revealed differences in d15N values between soil types, reflecting differences in vegetation. An eight-week laboratory incubation with constant temperature and gravimetric water content revealed that the shrubland soil had a higher potential rate of decomposition than the alfalfa field soil, even though alfalfa SOM had a lower C/N ratio. FTIR spectroscopy of the bulk soils suggests that the increased recalcitrance of the alfalfa soils is due to insoluble SOC moieties. Water limitations likely allowed for greater accumulation of labile C in the shrubland soil. Additionally, decomposition in the subsoil (90-100 cm) of each site was limited by substrate quality rather than environmental conditions. Taken together these results suggest that converting a semi-arid shrubland into irrigated cropland significantly reduces soil organic C, likely decreasing the overall sink strength of these systems.