Conventional agriculture has drastically increased food production, utilizing high inputs of fertilizers and pesticides to maximize yields, but has been shown to pollute water resources through nitrate leaching and contribute to climate change through greenhouse gas (GHG) emissions. Production agroforestry is a potentially transformative cropping system that utilizes multiple woody and herbaceous species grown together to produce food while also providing important ecological services such as carbon sequestration, and reductions in nutrient runoff and GHG emissions. We aimed to quantify how fertilization affects the tradeoffs between the production of ecosystem services (i.e., yield, nutrient retention, and GHG mitigation) in production agroforestry systems. In 2017, we initiated a fertilization experiment at three Chinese chestnut (Castanea mollissima) dominated production agroforests at 2- 15 years since planting. Three fertilization treatments (unfertilized, ammonium sulfate, and chicken manure) were randomized by blocks in plots (4-5 trees) at each site (4 blocks per site). Fertilizer was applied each June, with application rates varying by farm based on stand age (28-170g nitrogen per chestnut tree). We measured soil GHG emissions (i.e. CO2, CH4, N2O) using the static chamber method and nutrient leaching using resin lysimeters buried at 50-cm. We only measured chestnut yield at the mature site.
Results/Conclusions
Surprisingly, fertilizer treatments did not affect soil greenhouse gas emissions (as CO2-equivalents) in production agroforestry plots (May 2017- Jan 2018; pooled flux F2,238 = 0.67, p=0.51). There are no main effects of fertilization on soil CO2 (F2,265 = 2.00, p=0.14), CH4 (F2,284 = 1.73, p=0.18) or N2O (F2,311 = 0.23, p=0.79) emissions. There is evidence of fertilizer effects on soil CO2 emissions (site x fertilizer; F4,265 = 5.61, p<0.001) at the marginal site, where CO2 flux increased from ammonium sulfate fertilized to unfertilized and organically fertilized. Resin lysimeter extracts are currently being analyzed to assess fertilization effects on nitrate leaching. The first year of fertilization (2017) did not enhance chestnut production at the mature site (F2,45 = 2.32, p=0.11). This may represent a delay in fertilizer effects on chestnut yield until the following full growing season because trees are known to utilize overwinter nutrient stocks for chestnut production or may initially invest added nutrients for vegetative growth. Taken together, we found no effects of short-term fertilization on ecosystem service production in chestnut based production agroforestry, suggesting efficient nitrogen use toward vegetative growth rather than nut production.