Gross N2O emission and gross N2O uptake in temperate cropland agroforestry and monoculture systems

Nitrous oxide (N2O) is a powerful greenhouse gas and also the remaining threat to the ozone layer. N2O emission is mainly from cropland accounting for 82% of the global N2O increase, which is of great concern for policymakers making strategies for mitigating N2O emissions. One of such strategies is agroforestry systems which integrate trees into cropland and are considered as environmentally-friendly ecosystems, in particular in greenhouse gas mitigation (e.g. N2O emissions). The net balance of N2O flux is constrained by gross N2O emissions and uptake. However, we are still struggling to fully understand the complexity of gross N2O emissions and uptake due to its spatial- and temporal variation. No systematic comparison of gross N2O fluxes was conducted between cropland agroforestry and monoculture. Besides, N2O produced and consumed are not only in topsoil but also in subsoil and there is lacking information about how gross N2O emissions and uptake vary at depths in different types of agroforestry systems. The first study aimed to assess the impact of land-use change on gross N2O emissions and uptake and their associated controls between cropland agroforestry and monoculture. The study was conducted at three sites in Germany, of which two sites had paired cropland agroforestry and monoculture on a loam Calcaric Phaeozem soil and a clay Vertic Cambisol soil, and one site was a cropland monoculture on a sandy Arenosol soil. Gross N2O emissions and uptake were monthly measured by using the 15N2O pool dilution technique over two growing seasons (2018 - 2019). Our results showed that soil gross N2O emissions from the area-weighted tree and crop rows in the agroforestry did not differ from monoculture. Nonetheless, the unfertilized tree rows showed the lowest gross N2O emissions. Although tree rows only occupied 20% in the agroforestry, annual gross N2O emissions in the top 5-cm soil decreased by 6% to 36% in the agroforestry compared to monoculture. Gross N2O emissions were influenced by soil mineral N, available C, 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. The agroforestry tree row increased annual gross N2O uptake in the top 5-cm soil by 27% to 42% compared to monoculture. In the tree row, soil gross N2O uptake correlated with nirK gene abundance which, in turn, was correlated with nosZ clade II that was related to low mineral N-to-available C ratios. The second study aimed to compare gross N2O emissions and uptake between riparian tree buffer and tree row of alley cropping system, and between depths (0 – 5 vs. 40 – 60 cm), and to elucidate their associated abiotic and biotic controls. This study was conducted at two contrasting agroforestry systems in Germany: riparian tree buffer and tree row of the alley cropping system. We quantified gross N2O emissions and uptake using the 15N2O pool dilution technique in early spring (April), spring (June), summer (August), and fall (October). Our results showed that riparian tree buffer had higher gross N2O emissions and uptake in topsoil (0 – 5 cm) than the tree row of alley cropping but such differences were not observed in subsoil (40 – 60 cm). Although gross N2O emissions and uptake did not differ between the two depths in each agroforestry system, we observed a hot moment, i.e. early spring, for gross N2O emissions in topsoil of riparian tree buffer, with a large source of N2O observed. Gross N2O emissions were mainly controlled by mineral N, biodegradable organic carbon, and water-filled pore space rather than microbial population size between the two agroforestry systems and depths. Gross N2O uptake in topsoil was driven by available carbon and nirK gene abundance across agroforestry systems. But subsoil showed a sink of N2O due to low mineral N. Gross N2O uptake in subsoil was affected by soil temperature in each agroforestry system, indicating positive feedback of global warming. Overall, this research provides new insights into mitigation of N2O emissions from soil to atmosphere after conversion of cropland monoculture to agroforestry and also provides field-based rates of gross N2O fluxes at depths in contrasting agroforestry systems. Our research provides the first year-round quantification of gross N2O emission and uptake using 15N2O pool dilution for cropland agroforestry and monoculture, with key implications for support on greenhouse gas regulation function for policy implementation of agroforestry. Our findings emphasize that adjusting the tree and crop areal coverages of agroforestry can further optimize the benefits of agroforestry in reducing emissions and increasing uptake of N2O in soils. As discussed in the synthesis chapter, future studies should increase the measurement frequency of gross N2O fluxes at depths to capture hot moments and spots, especially in the riparian tree buffer, and further better constrain the contribution of subsoil to the ecosystem N loss although this area is relatively small.

2022-03-03