Abstract
AbstractSemiarid ecosystems are susceptible to changes in dominant vegetation which may have significant implications for terrestrial carbon dynamics. The present study examines the distribution of organic carbon (OC) between particle size fractions in near‐surface (0–0.05 m) soil and the water erosion‐induced redistribution of particle‐associated OC over a grass‐shrub ecotone, in a semiarid landscape, subject to land degradation. Coarse (>2 mm) particles have comparable average OC concentrations to the fine (<2 mm) particles, accounting for ~24–38% of the OC stock in the near‐surface soil. This may be due to aggregate stabilization by precipitated calcium carbonate in these calcareous arid soils. Critically, standard protocols assuming that coarse fraction particles contain no OC are likely to underestimate soil OC stocks substantially, especially in soils with strongly stabilized aggregates. Sediment eroded from four hillslope scale (10 × 30 m) sites during rainstorm events was monitored over four annual monsoon seasons. Eroded sediment was significantly enriched in OC; enrichment increased significantly across the grass‐shrub ecotone and appears to be an enduring phenomenon probably sustained through the dynamic replacement of preferentially removed organic matter. The average erosion‐induced OC event yield increased sixfold across the ecotone from grass‐dominated to shrub‐dominated ecosystems, due to both greater erosion and greater OC enrichment. This erosional pathway is rarely considered when comparing the carbon budgets of grasslands and shrublands, yet this accelerated efflux of OC may be important for long‐term carbon storage potentials of dryland ecosystems.
Highlights
Drylands are extensive ecosystems, covering around 40% of the land surface and directly providing ecosystem services to some 2.4 billion people [Adeel et al, 2005; Reynolds et al, 2007]
The study site is located in the Mackenzie Flats of the Sevilleta National Wildlife Refuge (SNWR) in central New Mexico, USA (34°19’N, 106°42’W), experiencing a semi-arid climate with 256 mm mean annual precipitation of which ~60% falls during the summer 182 monsoon period
The results presented suggest that organic carbon (OC) enrichment can be an enduring phenomenon, at least at hillslope scales in semi-arid rangelands, and we believe that the preferential removal of OC may be sustained long-term by the dynamic replacement of organic matter (OM) via litter inputs via the soil surface [Harden et al, 1999; Li et al, 2007; Berhe et al, 2008; Doetterl et al, 2012]
Summary
Drylands are extensive ecosystems, covering around 40% of the land surface and directly providing ecosystem services to some 2.4 billion people [Adeel et al, 2005; Reynolds et al, 2007]. Dryland soils usually contain only small amounts of organic carbon (OC) per-unit-area, their extent and low turnover rates means they contain an estimated 10-27% of the OC stock in terrestrial soils [Safriel et al, 2005; Finch, 2012]. It has been argued that dryland ecosystems may contribute significantly to interannual variations in the global carbon cycle [Poulter et al, 2014]. The degradation of dryland ecosystem carbon storage capacity is estimated to release ~0.3 Pg C yr-1 to the atmosphere from terrestrial stocks [Adeel et al, 2005; Safriel et al, 2005], and significantly influences the global biogeochemical carbon cycle [Schlesinger et al, 1990; Qi et al, 2001; Poulter et al., 2014]. There is large uncertainty regarding the fate of eroded OC, some of which is released to the atmosphere [Van Oost et al, 2005; Lal and Pimentel, 2008]
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