Marife D. Corre

Monoculture croplands are a major source of global anthropogenic emissions of nitrous oxide (N 2 O), a potent greenhouse gas that contributes to ozone depletion. Agroforestry has the potential to reduce N 2 O emissions. Presently, there is no systematic comparison of soil N 2 O emissions between cropland agroforestry and monoculture systems in Central Europe. We investigated the effects of converting the monoculture cropland system into the alley cropping agroforestry system on soil N 2 O fluxes at three sites (each site has paired agroforestry and monoculture) in Germany, where agroforestry combined crop rows and poplar short‐rotation coppice (SRC). We measured soil N 2 O fluxes monthly over 2 years (March 2018–January 2020) using static vented chambers. Annual soil N 2 O emissions from agroforestry ranged from 0.21 to 2.73 kg N ha −1 year −1 , whereas monoculture N 2 O emissions ranged from 0.34 to 3.00 kg N ha −1 year −1 . During the rotation of corn crop, with high fertilization rates, agroforestry reduced soil N 2 O emissions by 9% to 56% compared to monocultures. This was mainly caused by low soil N 2 O emissions from the unfertilized agroforestry tree rows. Soil N 2 O fluxes were predominantly controlled by soil mineral N in both agroforestry and monoculture systems. Our findings suggest that optimized fertilizer input will further enhance the potential of agroforestry for mitigating N 2 O emissions.

Conversion of monoculture to agroforestry (integrating trees with crops) is promoted as a promising management in reducing N 2 O emissions from croplands. How agroforestry influences gross N 2 O emission (N 2 O + N 2 from N 2 O reduction) and uptake (N 2 O reduced to N 2 ) compared to monoculture is unknown. We used the 15 N 2 O 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 N 2 O emissions ( P ≤ 0.002). Although tree rows occupied only 20% in agroforestry, gross N 2 O emissions tended to decrease by 6–36% in agroforestry (0.98–1.02 kg N 2 O‐N ha −1 yr −1 ) compared to monoculture (1.04–1.59 kg N 2 O‐N ha −1 yr −1 ). Gross N 2 O emissions were influenced by soil mineral N, soil respiration, and moisture content rather than by denitrification gene abundance. Soil gross N 2 O uptake was highest in the tree row and decreased with distance into crop rows ( P = 0.012). The agroforestry tended to increase gross N 2 O uptake by 27–42% (0.38–0.44 kg N 2 O‐N ha −1 yr −1 ) compared to monoculture (0.30–0.31 kg N 2 O‐N ha −1 yr −1 ). In tree rows, soil gross N 2 O uptake correlated with nirK gene abundance which was indirectly influenced by the low mineral N‐to‐soil CO 2 ‐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 N 2 O in soils.

Experimental evidence of nutrient limitations on primary productivity in Afrotropical forests is rare and globally underrepresented yet are crucial for understanding constraints to terrestrial carbon uptake. In an ecosystem‐scale nutrient manipulation experiment, we assessed the early responses of tree growth rates among different tree sizes, taxonomic species, and at a community level in a humid tropical forest in Uganda. Following a full factorial design, we established 32 (eight treatments × four replicates) experimental plots of 40 × 40 m each. We added nitrogen (N), phosphorus (P), potassium (K), their combinations (NP, NK, PK, and NPK), and control at the rates of 125 kg N ha −1 year −1 , 50 kg P ha −1 year −1 and 50 kg K ha −1 year −1 , split into four equal applications, and measured stem growth of more than 15,000 trees with diameter at breast height (dbh) ≥1 cm. After 2 years, the response of tree stem growth to nutrient additions was dependent on tree sizes, species and leaf habit but not community wide. First, tree stem growth increased under N additions, primarily among medium‐sized trees (10–30 cm dbh), and in trees of Lasiodiscus mildbraedii in the second year of the experiment. Second, K limitation was evident in semi‐deciduous trees, which increased stem growth by 46% in +K than –K treatments, following a strong, prolonged dry season during the first year of the experiment. This highlights the key role of K in stomatal regulation and maintenance of water balance in trees, particularly under water‐stressed conditions. Third, the role of P in promoting tree growth and carbon accumulation rates in this forest on highly weathered soils was rather not pronounced; nonetheless, mortality among saplings (1–5 cm dbh) was reduced by 30% in +P than in –P treatments. Although stem growth responses to nutrient interaction effects were positive or negative (likely depending on nutrient combinations and climate variability), our results underscore the fact that, in a highly diverse forest ecosystem, multiple nutrients and not one single nutrient regulate tree growth and aboveground carbon uptake due to varying nutrient requirements and acquisition strategies of different tree sizes, species, and leaf habits.