- مبلغ: ۸۶,۰۰۰ تومان
- مبلغ: ۹۱,۰۰۰ تومان
Climate and land use models predict tropical deforestation and conversion to cropland will produce a large flux of soil carbon (C) to the atmosphere from accelerated decomposition of soil organic matter (SOM). However, the C flux from the deep tropical soils on which most intensive crop agriculture is now expanding remains poorly constrained. To quantify the effect of intensive agriculture on tropical soil C, we compared C stocks, radiocarbon and stable C isotopes to 2 m depth from forests and soybean cropland created from former pasture in Mato Grosso, Brazil. We hypothesized that soil disturbance, higher soil temperatures (+2C), and lower OM inputs from soybeans would increase soil C turnover and deplete C stocks relative to nearby forest soils. However, we found reduced C concentrations and stocks only in surface soils (0-10 cm) of soybean cropland compared with forests, and these differences could be explained by soil mixing during plowing. The amount and Δ14C of respired CO2 to 50 cm depth were significantly lower from soybean soils, yet CO2 production at 2 m deep was low in both forest and soybean soils. Mean surface soil δ13C decreased by 0.5‰ between 2009 and 2013 in soybean cropland, suggesting low OM inputs from soybeans. Together these findings suggest: 1) soil C is relatively resistant to changes in land use, and 2) conversion to cropland caused a small, measurable reduction in the fast-cycling C pool through reduced OM inputs, mobilization of older C from soil mixing, and/or destabilization of SOM in surface soils.
In summary, conversion of seasonally dry tropical forests to pasture and cropland in the southern Amazon region of cropland expansion led to only small changes in soil C dynamics. While organic inputs from African pasture grasses such as Brachiaria spp. can meet or exceed those from forest trees, our data suggested that inputs from soybeans in the last decade must have been low relative to those from trees or pasture grasses under forest or pasture. Differences in C storage below the top 10 cm were not detected and mixing to 50 cm depth could account for some differences in both C storage and bulk soil Δ14C between forest and soybean cropland soils. However, lower rates of CO2 production and lower Δ14C of CO2 indicated that there was a loss of young C or destabilization of old C in the top 50 cm. With continued low SOM inputs from soybeans and hotter local and regional temperatures, further reductions in C stocks would be possible if these large C pools were responsive. However, we found that C stocks in these soils were mostly resistant to these drivers and that C stock changes in the coming decades are likely to be modest if similar crop management practices are used. Programs like Brazil's Low-Carbon Agriculture Plan that provides lines of credit for following best agricultural management practices can help reduce C losses from expanding tropical cropland in the Amazon (Angelo, 2012; de Moraes Sá et al., 2017).