Light and heavy fractions of soil organic matter in response to climate warming and increased precipitation in a temperate steppe

Abstract

Soil is one of the most important carbon (C) and nitrogen (N) pools and plays a crucial role in ecosystem C and N cycling. Climate change profoundly affects soil C and N storage via changing C and N inputs and outputs. However, the influences of climate warming and changing precipitation regime on labile and recalcitrant fractions of soil organic C and N remain unclear. Here, we investigated soil labile and recalcitrant C and N under 6 years' treatments of experimental warming and increased precipitation in a temperate steppe in Northern China. We measured soil light fraction C (LFC) and N (LFN), microbial biomass C (MBC) and N (MBN), dissolved organic C (DOC) and heavy fraction C (HFC) and N (HFN). The results showed that increased precipitation significantly stimulated soil LFC and LFN by 16.1% and 18.5%, respectively, and increased LFC:HFC ratio and LFN:HFN ratio, suggesting that increased precipitation transferred more soil organic carbon into the quick-decayed carbon pool. Experimental warming reduced soil labile C (LFC, MBC, and DOC). In contrast, soil heavy fraction C and N, and total C and N were not significantly impacted by increased precipitation or warming. Soil labile C significantly correlated with gross ecosystem productivity, ecosystem respiration and soil respiration, but not with soil moisture and temperature, suggesting that biotic processes rather than abiotic factors determine variations in soil labile C. Our results indicate that certain soil carbon fraction is sensitive to climate change in the temperate steppe, which may in turn impact ecosystem carbon fluxes in response and feedback to climate change.

Figure 1. Effects of warming and increased precipitation on soil total C and N (TC, TN) (a), light fraction C and N (LFC, LFN) (b), heavy fraction C and N (HFC, HFN) (c), LFC∶HFC ratio and LFN∶HFN ratio (d) (means ± SE).

C, control; W, warming; P, increased precipitation; WP, warming plus increased precipitation.

Figure 2. Effects of warming and increased precipitation on ratios of soil C∶N (TC∶TN), light fraction C∶N (LFC∶LFN) and heavy fraction C∶N (HFC∶HFN) (mean ± SE).

See Fig. 1 for abbreviations.

Figure 3. Effects of warming and increased precipitation on soil microbial biomass C and N (MBC, MBN) and soil dissolved organic C (DOC) (means ± SE).

See Fig. 1 for abbreviations.

Figure 4. Linear correlations between GEP and light fraction C (a) or N (b), MBC (c) and DOC (d) across all the 24 subplots.

GEP, gross ecosystem productivity, whose values were the yearly mean values from 2005 to 2009.

Figure 5. Linear correlations between carbon flux (ER or SR) and light fraction C (a, b) or N (c, d) across all the 24 subplots.

ER, ecosystem respiration; SR, soil respiration. The values of ER and SR were the yearly mean values from 2005 to 2009.

Figure 6. Linear correlations between carbon flux (ER or SR) and MBC (a, b) or DOC (c, d) across all the 24 subplots.

ER, ecosystem respiration; SR, soil respiration; MBC, microbial biomass C; DOC, soil dissolved organic C. The values of ER and SR were the yearly mean values from 2005 to 2009.