Reduced Soil Gross N₂O Emission Driven by Substrates Rather Than Denitrification Gene Abundance in Cropland Agroforestry and Monoculture

Abstract

Conversion of monoculture to agroforestry (integrating trees with crops) is promoted as a promising management in reducing N2O emissions from croplands. How agroforestry influences gross N2O emission (N2O + N2 from N2O reduction) and uptake (N2O reduced to N2) compared to monoculture is unknown. We used the 15N2O pool dilution technique to quantify these processes using soil cores (top 5 cm) incubated in the field with monthly measurements over two growing seasons (2018–2019) at two sites (each with paired agroforestry and monoculture) and one site with monoculture only. The unfertilized tree rows showed the lowest gross N2O emissions (P ≤ 0.002). Although tree rows occupied only 20% in agroforestry, gross N2O emissions tended to decrease by 6–36% in agroforestry (0.98–1.02 kg N2O‐N ha−1 yr−1) compared to monoculture (1.04–1.59 kg N2O‐N ha−1 yr−1). Gross N2O emissions were influenced by soil mineral N, soil respiration, and moisture content rather than by denitrification gene abundance. Soil gross N2O uptake was highest in the tree row and decreased with distance into crop rows (P = 0.012). The agroforestry tended to increase gross N2O uptake by 27–42% (0.38–0.44 kg N2O‐N ha−1 yr−1) compared to monoculture (0.30–0.31 kg N2O‐N ha−1 yr−1). In tree rows, soil gross N2O uptake correlated with nirK gene abundance which was indirectly influenced by the low mineral N‐to‐soil CO2‐C ratio. Adjusting the tree and crop areal coverages of agroforestry and optimizing fertilization can further augment the benefits of agroforestry in reducing emission and increasing uptake of N2O in soils.

Abundance, Diversity, and Function of Soil Microorganisms in Temperate Alley-Cropping Agroforestry Systems: A Review

“…The authors reported an increase from 52 to 81 in MBC and 31 to 63% in MBN in the tree rows as compared to multiple distances in the crop rows [ 39 ]. Three years later, Luo and coworkers verified these findings [ 45 ]. At two sites, the authors were able to show a gradual increase in MBC from the middle of the crop rows (365 mg kg −1 ) to the tree rows (572 mg kg −1 ) [ 45 ].…”

“…Three years later, Luo and coworkers verified these findings [ 45 ]. At two sites, the authors were able to show a gradual increase in MBC from the middle of the crop rows (365 mg kg −1 ) to the tree rows (572 mg kg −1 ) [ 45 ]. Similarly, MBN was 46 to 69% greater in the tree rows than in the middle of the crop rows [ 45 ].…”

“…Microorganisms inhabiting the soil of temperate alley-cropping agroforestry systems have been studied using a variety of methods ranging from microscopy techniques to modern high-throughput sequencing approaches. Among the most frequently applied methods are chloroform fumigation extraction (CFE) for the determination of the microbial biomass [ 27 , 35 , 36 , 39 , 40 , 41 , 42 , 43 , 44 , 45 ] and the measurement of potential enzyme activities [ 36 , 39 , 46 , 47 , 48 , 49 , 50 , 51 ]. Other popular methods to characterize soil microbial communities in temperate agroforestry systems include substrate-induced respiration [ 27 , 39 , 46 , 52 ] and phospholipid fatty acids (PLFA) analysis [ 27 , 40 , 50 , 53 ].…”

“…At two sites, the authors were able to show a gradual increase in MBC from the middle of the crop rows (365 mg kg −1 ) to the tree rows (572 mg kg −1 ) [ 45 ]. Similarly, MBN was 46 to 69% greater in the tree rows than in the middle of the crop rows [ 45 ]. These observations agree with findings from cherry-based intercropping systems [ 41 ].…”

“…Few studies have quantified functional genes involved in soil-N cycling (N 2 fixation, nitrification, and denitrification) in agroforestry systems using real-time PCR [ 32 , 38 , 45 , 54 ]. From 2020 to 2021, two studies showed that the abundance of the N 2 -fixation gene nifH was increased in the tree row as compared to the crop row and adjacent cropland monoculture at one of their three study sites with poplar-based alley-cropping systems.…”