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Keywords:

  • carbon and nitrogen dynamics;
  • microbial biomass;
  • qCO2;
  • tropical cropland;
  • wetting–drying cycle

Abstract

The wetting–drying cycles of soil primarily drive carbon (C) and nitrogen (N) dynamics in tropical monsoon climates. We evaluated the in situ short-term C and N dynamics and the effect of soil texture during a wetting–drying cycle in relation to hourly microbial dynamics. Two croplands of differing soil textures (clay [THc] and sand [THs]) in Thailand were used for the experiment. Hourly measurements of soil CO2 efflux and inorganic nitrogen (Inorg-N) were conducted and we determined fluctuations in the in situ microbial biomass (In-situ-MB) and in situ microbial activity (In-situ-qCO2) after the application of a simulated rainfall (W plot) and a rainfall/glucose (WG plot) treatment. The rewetting of dry soil led to a C flush, which finished within 50–120 h because of rapid soil drying at both sites. Comparing the microbial dynamics in the THc-W and THs-W plots, it is clear that the rainfall treatment predominantly increased In-situ-qCO2 in the THc-W plot, whereas it increased In-situ-MB in the THs-W plot. These different microbial dynamics resulted in different C and N dynamics; that is, the cumulative soil CO2 efflux for the first 100 h after the treatment was effectively greater in the THs-W plot (3.4 g C m−2) than in the THc-W plot (2.8 g C m−2). In addition, distinct N-mineralization associated with a decreasing In-situ-MB was observed only in the THs-W plot, although this was not the case in the THc-W plot. Hence, we concluded that rainfall events should play a more important role in the C and N dynamics in sandy soils compared with clayey soils because of different microbial dynamics after rewetting of a dry soil.