Multifunctionality of temperate alley-cropping agroforestry outperforms open cropland and grassland

Modern agricultural systems prioritize high productivity and profit, but externalized costs such as soil degradation, water pollution, GHG emissions, and biodiversity loss are borne by society. There is growing recognition that agriculture must also deliver ecosystem functions that support biodiversity, carbon sequestration, and reduced environmental pollution. Agroforestry is a promising agro-ecological management approach and is being considered for financial incentives in policies such as the European Common Agricultural Policy. A key need is comprehensive evaluation of multifunctionality—the simultaneous provision of multiple ecosystem functions—of agroforestry relative to conventional monocultures. This study addresses that gap by comparing temperate alley-cropping agroforestry (rows of crops or grass alternated with rows of short-rotation trees) with open croplands and open grasslands in a single, replicated, multidisciplinary study across Germany. We quantified 47 indicators covering seven ecosystem functions in croplands and 16 indicators covering four functions in grasslands over four years at five sites. We hypothesized that alley-cropping agroforestry would enhance beneficial ecosystem functions and overall multifunctionality relative to open croplands and grasslands.

Prior studies have reported individual benefits of temperate agroforestry, including increases in soil organic carbon, enhanced diversity of soil organisms, improved nutrient utilization efficiency, greater wind erosion resistance, and reduced nitrate leaching. Meta-analyses suggest temperate agroforestry can increase SOC over longer timescales. However, systematic, multi-function assessments comparing temperate agroforestry with open cropland or grassland within a single, replicated field-based study have been lacking, motivating the comprehensive multifunctionality assessment performed here.

Study design: Five sites in Germany were studied: three cropland systems (Thuringia: Calcaric Phaeozem; Lower Saxony: Vertic Cambisol; Brandenburg: Gleyic Cambisol) and two grassland systems (Lower Saxony: Histosol and Anthrosol). Each site included paired alley-cropping agroforestry (12 m tree rows alternating with 48 m crop or grass rows) and adjacent open cropland/grassland. Replication: Croplands had four replicate plots per management (3 sites × 2 systems × 4 plots = 24 plots); grasslands had three replicate plots per management (2 sites × 2 systems × 3 plots = 12 plots). Agroforestry was established between 2007–2010. Management followed customary practices (conventional rotations; mineral fertilizers and agrochemicals in crop rows; no fertilization of tree rows; trees harvested every 4–7 years for bioenergy). Spatial sampling: At agroforestry plots, measurements were taken in tree rows and at multiple distances into the crop/grass rows (e.g., 1 m, 4 m, 7 m, 24 m) to capture edge-to-center gradients; values were area-weighted by the proportion of the 12 m tree and 48 m crop/grass rows. Indicators: - Provision of food, fiber, and fuel (all sites): Grain yield (combine harvest), grass biomass (frame harvest), 1000-grain weight, crop/grass quality (crude protein, starch, fat, fiber via NIRS/Soxhlet/Dumas/Ewers/Weender), phytopathogen abundances (qPCR for Fusarium spp., Verticillium longisporum, Leptosphaeria maculans). - Carbon sequestration (all sites): Tree aboveground net primary production (woody biomass via DBH allometries plus leaf litterfall), combined fine root density (0–100 cm), SOC stocks (0–30 cm; CN analyzer; bulk density for equivalent mass). - Soil nutrient cycling (all sites): Gross N mineralization (15N pool dilution, top 5 cm), plant-available P (resin + bicarbonate P; monthly growing season), plant-available K (NH4Cl extraction, 0–30 cm), base saturation and effective CEC (sum of exchangeable cations), nifH gene abundance (qPCR). - Habitat for soil biological activity (all sites): Earthworm biomass (hand-sorting soil monoliths), microbial biomass C and N (chloroform fumigation-extraction), bacterial and fungal abundances (qPCR), β-glucosidase activity (fluorometric assay). - Soil greenhouse gas abatement (cropland only): Net soil N2O and CH4 fluxes (monthly static chambers, Mar 2018–Feb 2019; GC analysis), gross N2O uptake (15N2O pool dilution). - Water regulation (cropland only): Actual evapotranspiration (eddy covariance; annual sums), saturated hydraulic conductivity Ks (lab cores; Darcy equation; normalized to 20°C), modeled change in soil water storage (Expert-N; top 60 cm), nutrient leaching fluxes (drainage from Expert-N combined with lysimeter concentrations for N, P, S, Na, K, Ca, Mg). - Erosion resistance (one cropland site, Gleyic Cambisol): Wind speed reduction (LES simulations validated with measurements; three wind directions; 0.5 m height) and number of days with wind speeds >5 m s−1 as wind erosion risk indicator. Data processing and statistics: For agroforestry, spatially area-weighted plot values were computed. Indicators were z-standardized across plots and sites; indicators where high values denote undesirable effects (pathogen abundance, nutrient leaching, soil GHG fluxes, wind speed) were inverted. Differences in ecosystem functions (aggregated from multiple indicators) between management systems were tested with linear mixed-effects models (management as fixed effect; indicators and plots across sites as random effects), with indicator × management interactions evaluated. Spearman rank correlations among ecosystem functions were computed within each management system and for relative changes due to conversion. Analyses were conducted in R 4.0.4.

• Yield and quality: Conversion to alley-cropping did not affect crop yield or grass biomass per cropped/grass-allocated area. Yield reductions near tree rows were compensated by increases toward crop row centers. Crop quality partially improved in cropland agroforestry: higher wheat crude starch, higher canola crude protein, and higher canola 1000-grain weight (P ≤ 0.03). Most phytopathogenic fungi were unaffected, except Verticillium longisporum in canola was reduced in agroforestry (P = 0.03). - Carbon sequestration: Strongly increased in both cropland and grassland agroforestry (P < 0.01), driven by high tree net primary production. Fine root density increased in cropland agroforestry (P < 0.01). SOC stocks (≤9-year-old systems) did not differ from open systems (P > 0.05). - Habitat for soil biological activity (cropland): Improved (P = 0.03) with greater earthworm biomass (P < 0.01) and larger soil bacterial and fungal populations (P ≤ 0.03). In grasslands, habitat function was comparable between agroforestry and open grassland (P > 0.05). - Erosion resistance (cropland): Wind speed and number of days with wind erosion risk were reduced due to tree rows, significantly increasing wind erosion resistance (P < 0.01). - Soil nutrient cycling, soil GHG abatement, and water regulation: No overall change detected after conversion in either croplands or grasslands (P > 0.05), except higher plant-available P in grassland agroforestry (P = 0.02) and greater gross soil N2O uptake in cropland agroforestry (P = 0.01). - Multifunctionality: Alley-cropping increased overall multifunctionality compared to open cropland; no indicators showed a decrease under agroforestry. - Correlations: No correlations among functions within cropland agroforestry; positive correlation between carbon sequestration and nutrient cycling in grassland agroforestry. In open cropland, water regulation correlated negatively with nutrient cycling and habitat for soil biological activity; upon conversion to cropland agroforestry, a trade-off emerged between water regulation and nutrient cycling. Conversion of open grassland to agroforestry yielded a positive correlation between nutrient cycling and habitat for soil biological activity.

The study confirms that temperate alley-cropping agroforestry can enhance key ecosystem functions—carbon sequestration, soil biological habitat, and wind erosion resistance—without reducing yields across the cropped area. Improved functions address global challenges: increased carbon inputs via trees and roots support climate regulation; enhanced earthworm and microbial populations bolster soil health and may cascade to higher trophic levels; and reduced wind erosion mitigates losses of SOC, nutrients, and productivity. Lack of differences in SOC stocks likely reflects the relatively young age (≤9 years) of systems; other studies indicate SOC gains in ≥15-year-old agroforestry, suggesting increases with time. Functions tied to fertilization (nutrient cycling, GHG abatement, water regulation) did not improve, likely due to uniformly high fertilizer inputs in both agroforestry crop rows and open cropland. Despite robust spatial (distance-to-tree) and temporal (monthly over key periods) sampling, annual soil GHG abatement did not differ, supporting the conclusion that high fertilization masks potential benefits such as nutrient recycling via litterfall and deep root capture. Nevertheless, agroforestry enhanced gross soil N2O uptake (driven by unfertilized tree rows with favorable C:N substrate ratios), demonstrating a mitigation pathway. Trade-offs were identified: in croplands, water regulation negatively related to nutrient cycling after conversion, while grasslands showed synergy between nutrient cycling and biological habitat. Financial and policy dimensions are critical: although gross margins can increase with alley-cropping, high establishment costs and irregular tree biomass income are barriers; surveyed farmers indicated substantial upfront and annual support would be needed. Aligning subsidies with environmental performance and incentivizing optimized nutrient management could unlock agroforestry’s full multifunctional potential.

Alley-cropping agroforestry with short-rotation trees enhances multifunctionality compared to open croplands and improves carbon sequestration in both cropland and grassland systems without compromising yields. Key benefits include stronger carbon inputs, richer soil biological habitat, and reduced wind erosion risk. Realizing full environmental gains—particularly in nutrient cycling, GHG abatement, and water regulation—requires optimizing nutrient management (reducing excessive fertilization, precision placement, and crop diversification with shade-tolerant species). Policy instruments should prioritize conversion of open cropland to agroforestry and couple financial support with practices that lower external environmental costs. Future research should include long-term evaluations of reduced fertilization in agroforestry, spatiotemporally targeted nutrient inputs, crop diversification strategies relative to tree age/height, and extended monitoring to capture SOC stock changes as systems mature.

• System age: SOC stock changes may require longer timeframes; studied systems were ≤9 years old, potentially underestimating long-term carbon gains. - Spatial scope: Wind erosion resistance indicators were quantified at one cropland site (with validation), which may limit generalization across all sites, though tree-crop row configurations were similar elsewhere. - Functional coverage: Some ecosystem services (air quality regulation, natural hazard regulation, pest/disease regulation as stand-alone functions) were not assessed due to scale constraints; pathogen data were included under provisioning. - Management context: High, uniform fertilization typical of conventional management likely masked potential improvements in nutrient-related functions; results may differ under optimized nutrient regimes. - GHG flux variability: Despite robust spatiotemporal sampling and modeling, inherent variability in N2O and CH4 fluxes remains a source of uncertainty. - Geographic generalizability: Results are from Germany across specific soil types and climates; extrapolation to other regions should consider local conditions and management.