Abstract
The conversion of forest to agricultural soils is a widespread activity in tropical systems, and its link to nitrous oxide (N2O) fluxes and nitrogen cycling gene abundance is relevant to understand environmental drivers that may interact with climate change. A current challenge to estimating N2O emissions from land use conversion is an incomplete understanding of crop-specific impacts on denitrifier communities and the N2O fluxes driven by differences in the above- and below-ground inputs with crop type. To address this knowledge gap in tree crops, we evaluated N2O fluxes and denitrification gene abundance and their relationships with soil and plant residue characteristics in citrus and eucalyptus plantations in the field and in soil incubations. We found that the accumulated N2O fluxes from soil were lower for the two agricultural field sites than those for their adjacent forest sites in dry and wet seasons. The N2O fluxes were higher in the wet season, and this seasonal difference persisted even when the soils collected from both seasons were incubated under the same moisture and temperature conditions in the lab for 30 days. Increased N2O fluxes in the wet season were accompanied by an increase in soil nirK and nosZ gene abundance, the dissolved organic carbon (DOC) concentration, and the total soil carbon (C) and nitrogen (N) content. In turn, the abundance of denitrifiers, as indicated by nirK, nirS and nosZ gene copy numbers, showed a low but significant positive correlation with soil bulk density. Our results suggest that soil moisture, leaf litter and crop residues influence the seasonal differences in both N2O fluxes and abundance of denitrifiers in citrus- and eucalyptus-cultivated soils, likely through effects on soil physicochemical characteristics. These findings highlight the overwhelming role of environmental drivers that can make investigating microbial drivers difficult in the field and open the possibility for a better understanding of N cycling processes in tropical soils based on paired field- and incubation-based experimentation.
Highlights
Many gaseous nitrogen (N) emissions from soils are a consequence of soil microbial processes (Hallin et al, 2018)
We evaluated seasonal patterns in N2O fluxes and denitrification gene abundance and their relationships with soil and plant residue characteristics in citrus and eucalyptus plantations
P-values were assigned by Satterthwatie approximation in models fit with restricted maximum likelihood (REML)
Summary
Many gaseous nitrogen (N) emissions from soils are a consequence of soil microbial processes (Hallin et al, 2018). A high demand for wood material and orange exports causes eucalyptus and citrus crops to play important roles in the agricultural sector in Brazil (Neves et al, 2010; Cavararo, 2014). The impacts of N additions on N2O emissions have been studied most often in natural forests and rarely in tropical plantations (Martinson et al, 2013; Mori et al, 2013; Zhang et al, 2014). The effects of agricultural management of tree crops (e.g., citrus orchards and eucalyptus plantations) on the N2O balance remain unclear. It is critical to understand the sensitivity of the microbial communities that regulate ecosystem N cycling to the soil, and the changes in both environmental and crop-specific conditions that accompany natural forest loss via conversion to agriculture
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