Soil CO_2 Emissions from Northern Andean Paramo Ecosystems: Effects of Fallow Agriculture

摘要

The effects of fallow agriculture on soil organic matter (SOM) dynamics and CO_2 emissions were assessed in the tropical Andean paramo ecosystem. Possible changes during the cultivation-fallow cycle were monitored in four areas of the Quebrada Pinuelas valley (Venezuela). Uncultivated soils and plots at different stages of a complete cultivation-fallow cycle were incubated, and SOM mineralization kinetics was determined. Soils exhibited a low SOM mineralization activity, total CO_2 evolved never reaching 3% of soil carbon, pointing to a stabilized SOM. Potential soil CO_2 effluxes differed significantly according to their plot aspect: northeast (NE)-aspect soils presented higher CO_2effluxes than southwest (SW)-aspect soils. SoilCO_2 emissions decreased after ploughing as compared to virgin paramo; low CO_2 effluxes were still observed during cropping periods, increasing progressively to reach the highest values after 4-5 y of fallow. In all cases, experimental C mineralization data was fitted to a double exponential kinetic model. High soil labile C pool variability was observed, and two different trends were identified: NE-oriented soils showed more labile C and a wider range of values than SW-facing soils. Labile C positively correlated with CO_2effluxes and negatively with its instantaneous mineralization rate. The instantaneous mineralization rate of the recalcitrant C pool positively correlated with %C evolved as CO_2and negatively with soil C and Al_2O_3 contents, suggesting the importance of aluminum on SOM stability. The CO_2 effluxes from these ecosystems, as well as the proportion of soil C released to the atmosphere, seem to depend not only on the size of the labile C pool but also on the accessibility of the more stabilized SOM. Therefore, fallow agriculture produces moderate changes in SOM quality and temporarily alters the CO_2 emission capacity of these soils.