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Crop cultivation and intensive grazing affect organic C pools and aggregate stability in arid grassland soil

Li, X-G, Wang, Z-F, Ma, Q. and Li, F-M (2007) Crop cultivation and intensive grazing affect organic C pools and aggregate stability in arid grassland soil. Soil and Tillage Research, 95 (1-2). pp. 172-181.

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The effects of cultivation and overgrazing on soil quality in arid regions have been rarely addressed. This study investigated the roles of cropping and grazing in soil organic C pools and aggregate stability at 0-20 cm depth by comparing conventional grazing (non-fenced ever), intensive grazing (fenced for 22 years) and cropping (cultivated for 40 years) in the arid Hexi Corridor of northwestern China. Total soil organic C (TOC) under non-fenced grazing was 21.6 g kg-1 (or 52.9 Mg ha-1), which was 19.9% (or 13.2% mass per area) lower than that under fenced grazing, because of lower stable organic C fraction (<0.05 mm) (i.e., 15.2 g kg-1 or 37.4 Mg ha-1 in non-fenced versus 19.2 g kg-1 or 44.5 Mg ha-1 in fenced grazing). Cropping had similar TOC concentration but 15.7% less TOC mass per area compared with non-fenced grazing mainly due to a decrease in coarse organic C (2-0.1 mm) (i.e., 4.1 g kg-1 or 10.1 Mg ha-1 in non-fenced versus 2.9 g kg-1 or 6.0 Mg ha-1 in cropping). Non-fenced grazing produced 1.49, 1.17 and 0.19 g kg-1 of soil carbohydrate C extracted by concentrated acid, diluted acid and hot water, respectively. The three carbohydrate C fractions were increased by 21.5, 14.5 and 15.8% under fenced grazing but lowered by 12.8, 18.8 and 21.1% under cropping, respectively. Soil mineralized C after 51-day incubation was the highest under fenced grazing followed by non-fenced grazing, and the lowest under cropping. Percentage of water-stable aggregates (>0.25 mm) in total aggregates and mean weight diameter were 15% and 0.28 mm under cropping, significantly lower than 65% and 3.11 mm under non-fenced grazing and 65% and 2.84 mm under fenced grazing. The aggregates of >1 mm were almost entirely demolished under cropping when subjected to wet sieving. Reduction of soil carbohydrates under cropping was closely related to the decline in aggregate water-stability. The negative effects of cropping on soil organic C pool and aggregate water-stability may suggest that cropping on this arid grassland is not sustainable unless no-tillage is adopted. In favor of increasing soil carbohydrates and maintaining soil aggregation, fenced-grazing would be a better option than cropping and non-fenced grazing for the management of arid grasslands.

Item Type: Journal Article
Publisher: Elsevier
Copyright: © 2007 Elsevier B.V.
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