Abstract
Soil nitrous oxide (N2O) emissions are influenced by land use adjustment and management practices. To meet the increasing socioeconomic development and sustainable demands for food supply, forestland conversion to cropland occurs around the world. However, the effects of forestland conversion to cropland as well as of tillage and fertilization practices on soil N2O emissions are still not well understood, especially in subtropical regions. Therefore, field experiments were carried out to continuously monitor soil N2O emissions after the conversion of forestland to cropland in a subtropical region in Southwest China. One forestland site and four cropland sites were selected: forestland (CK), short-term croplands (tillage with and without fertilization, NC-TF and NC-T), and long-term croplands (tillage with and without fertilization, LC-TF and LC-T). The annual cumulative N2O flux was 0.21 kg N ha−1 yr−1 in forestland. After forestland conversion to cropland, the annual cumulative N2O flux significantly increased by 76‒491%. In the short-term and long-term croplands, tillage with fertilization induced cumulative soil N2O emissions that were 94% and 235% higher than those from tillage without fertilization. Fertilization contributed 63% and 84% to increased N2O emissions in the short-term and long-term croplands, respectively. A stepwise regression analysis showed that soil N2O emissions from croplands were mainly influenced by soil NO3− and NH4+ availability and WFPS (water-filled pore space). Fertilization led to higher soil NH4+ and NO3− concentrations, which thus resulted in larger N2O fluxes. Thus, to reduce soil N2O emissions and promote the sustainable development of the eco-environment, we recommend limiting the conversion of forestland to cropland, and meanwhile intensifying the shift from grain to green or applying advanced agricultural management practices as much as possible.
Highlights
Nitrous oxide (N2O) has been recognized as an important non-CO2 greenhouse gas, with 298 times greater global warming potential than that of CO2 based on a 100-year time horizon [1]
The yield-scaled N2O emissions were 0.16 and 0.19 kg N Mg−1 grain in croplands after long-term cultivation (Table 3), which were similar to the results reported by Bayer et al [20] and Tang et al [39] in other subtropical regions
The yield-scaled N2O emissions were significantly higher in the newly converted croplands than in the long-term converted croplands. These results further indicate that the effect of land use conversion on soil N2O emissions should not be ignored in the initial years after conversion
Summary
Nitrous oxide (N2O) has been recognized as an important non-CO2 greenhouse gas, with 298 times greater global warming potential than that of CO2 based on a 100-year time horizon [1]. Agricultural soils are the largest N2O source, contributing about 60% of global anthropogenic N2O emissions [1,3]. The adjustment of land use is viewed as a crucial anthropogenic N2O emission source via its significant influences on N substrates and environmental conditions [1,9,12,13]. The existing studies focusing on the effects of land use change on N2O emissions mainly address rice paddy conversion to vegetable fields or citrus orchards [3,9,14], forest conversion to tea plantations [11,15], forest conversion to pasture or cropland [12], and cropland conversion to forestland [16,17].
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