Dissolved organic carbon and nutrient leaching in temperate alley-cropping agroforestry and open cropland

Abstract. Alley-cropping agroforestry with short-rotation coppice (AF) has been identified as a sustainable alternative to highly industrialised agricultural systems, enhancing ecosystem functions, including the regulation of water quality. However, there is limited knowledge regarding the leaching of dissolved organic carbon (DOC) and nutrients in AF and treeless open cropland (OC). In this study, we compared the leaching of mineral nitrogen, dissolved organic nitrogen (DON), DOC, potassium, and phosphorus in AF tree rows (AFtree) and AF crop rows (AFcrop) with OC across three sites with different soil textures in Germany over four years (2019–2022). The objectives were to (1) quantify the fluxes of DOC and nutrients in AF and OC systems and (2) identify the primary drivers of leaching in both systems. At each site, we sampled soil-pore water from AFtree, AFcrop (at 1 m, 7 m, and 24 m from the AFtree) and OC monthly using suction cup lysimeters installed at 60-cm depth and analysed for DOC and nutrient concentrations. AFtree was unfertilised whereas AFcrop and OC had the same conventional fertilisation rates. Drainage fluxes were estimated using the Expert-N water sub-model. Our results showed that, in loam Phaeozem and clay Cambisol soils with mature AF (established for at least 12 years with ≥ 5-year-old AFtree in the second rotation), AFtree had N leaching of 0–0.5 kg N ha^-1 yr^-1 whereas OC had 1–4 kg N ha^-1 cropping period^-1. Mature AF in the loam Phaeozem and clay Cambisol soils had two to 10 times lower NO₃⁻ leaching at 1 m than at 7 m and 24 m AFcrop. The effect of the AFtree on leaching was limited to ≤ 7 m AFcrop. Overall, AF (the area-weighted average of AFtree and AFcrop) did not differ from OC in DOC and nutrient leaching fluxes. An exception was observed in the sandy Arenosol soil during the first two years following AF establishment, when the overall AF exhibited higher NO₃^– and K leaching fluxes than OC, attributed to the large liquid manure application combined with soil disturbance from tree establishment and young tree roots. Across sites, precipitation and sand content were the dominant drivers of DOC and nutrient leaching in AFtree whereas precipitation was the only factor influencing leaching in AFcrop and OC. These results reinforce the importance of synchronising fertiliser input with the crop's demand in AFcrop as excessive fertilisation can override the tree root interception of nutrients at greater distances from the AFtree.