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

Current agricultural practices in industrialized countries prioritize high farm-level productivity and profit; however, they often neglect external costs such as soil degradation, water pollution, increased greenhouse gas (GHG) emissions, and biodiversity loss. Intensively managed cropland monocultures, while highly productive, have raised concerns about these detrimental environmental consequences. This has led to a growing awareness that modern agricultural systems should not only focus on high production but also on providing essential ecosystem functions and landscape features that support biodiversity, carbon sequestration, and reduce environmental pollution and soil degradation. Healthy soils and their functions are crucial for sustainable intensification of agriculture. Agroforestry is considered a promising approach for this. There is increasing interest in incorporating financial incentives linked to agroforestry's environmental performance compared to cropland monocultures. These incentives necessitate a comprehensive evaluation of ecosystem functions, including their capacity to provide multiple functions simultaneously (multifunctionality). This study addresses this knowledge gap by comparing the multifunctionality of temperate alley-cropping agroforestry (rows of crops or grass alternating with rows of short-rotation trees) to open croplands and grasslands. While individual studies have highlighted improvements in soil properties and ecosystem functions in temperate agroforestry (e.g., increased soil organic carbon, biodiversity, nutrient use efficiency, wind erosion resistance, reduced nitrate leaching), a lack of systematic comparisons of combined ecosystem functions across different systems within a single, multidisciplinary study using a replicated field-based design persists. This research aims to quantify the multifunctionality of alley-cropping agroforestry versus open croplands and grasslands in Germany across varying soil types and climatic conditions, hypothesizing that alley-cropping agroforestry will promote beneficial ecosystem functions and foster multifunctionality.

The literature review section supports the introduction by citing relevant studies demonstrating the positive and negative aspects of intensive agriculture and the potential benefits of agroforestry. Studies showing increased soil organic carbon, biodiversity improvements, enhanced nutrient use efficiency, and wind erosion reduction in agroforestry systems are referenced. However, the review highlights a critical gap: the lack of comprehensive, multi-functional comparisons of agroforestry with conventional croplands and grasslands using a robust experimental design. This gap in existing research provides a strong justification for the present study's design and methodology.

This study was conducted on three cropland and two grassland systems in Germany, each with alley-cropping agroforestry paired with open cropland or grassland. Replicate plots were established at each site, with measurements taken at various distances from the tree rows in agroforestry systems. The study spanned four years, analyzing 47 indicators across seven ecosystem functions in croplands and 16 indicators across four functions in grasslands. These functions included: provision of food, fiber, and fuel; carbon sequestration; soil nutrient cycling; habitat for soil biological activity; soil GHG abatement; water regulation; and erosion resistance. Specific indicators measured were tailored to each ecosystem function and included parameters such as crop yield and quality, tree biomass production, soil organic carbon stocks, earthworm biomass, microbial community composition and activity, greenhouse gas fluxes, evapotranspiration, nutrient leaching, and wind speed. Statistical analyses, primarily linear mixed-effects models, were employed to compare ecosystem functions between agroforestry and open cropland or grassland systems. Correlations between ecosystem functions were also assessed using Spearman rank correlation tests to identify potential trade-offs or synergies.

The study's key findings demonstrate that alley-cropping agroforestry significantly improved certain ecosystem functions compared to open croplands and grasslands without compromising crop yield per cropped area. Specifically, in cropland agroforestry, carbon sequestration, habitat for soil biological activity, and wind erosion resistance significantly improved (P ≤ 0.03). In grassland agroforestry, only carbon sequestration showed significant improvement (P < 0.01). However, in cropland agroforestry, soil nutrient cycling, soil GHG abatement, and water regulation did not show improvements, likely due to customary high fertilization rates. The analysis revealed that alley-cropping agroforestry increased overall multifunctionality compared to open croplands. The lack of improvement in certain functions within cropland agroforestry highlights the need for more efficient nutrient use and optimized fertilizer management. Detailed statistics on various indicators, including z-standardized values, are presented in the paper's figures and supplementary tables. Notably, although soil organic carbon stocks didn't differ significantly between agroforestry and open cropland initially, the authors anticipate increases over time.

The findings address the research question by demonstrating the enhanced multifunctionality of alley-cropping agroforestry. The significant improvements in carbon sequestration, soil biodiversity, and wind erosion resistance highlight the positive ecological impact of this agroforestry system. The lack of improvement in nutrient cycling and GHG abatement, however, points to the crucial role of optimized fertilizer management in maximizing the environmental benefits. High fertilization rates, a common practice in the study area, seem to mask potential positive effects from improved nutrient cycling within the agroforestry systems. The results have significant relevance to the field, supporting the promotion of agroforestry as a sustainable land management practice. However, the full potential of cropland agroforestry can only be realized through policy changes that incentivize more efficient nutrient use and reduced fertilization.

This study provides strong evidence that temperate alley-cropping agroforestry enhances multifunctionality compared to open cropland and grassland, improving carbon sequestration, soil biological activity, and wind erosion resistance. However, the study highlights the critical need for optimized nutrient management to fully realize the environmental benefits of agroforestry. Future research should focus on evaluating strategies to reduce fertilizer application rates in agroforestry systems while maintaining or even enhancing crop yields, possibly through precision farming techniques or diversification of crop species. Policy changes that provide financial incentives for agroforestry establishment and improved nutrient management practices are crucial for widespread adoption.

The study's relatively short duration (four years for most data) might not fully capture the long-term effects of agroforestry on some ecosystem functions, such as soil organic carbon accumulation. The study was limited geographically to specific regions of Germany and may not be directly generalizable to other regions with different climatic conditions and soil types. The customary high fertilization rates used in the conventional cropland management might have masked some potential positive effects of agroforestry on soil nutrient cycling and GHG abatement. Finally, the economic analysis is limited and doesn't fully address all the economic considerations associated with the adoption of agroforestry systems.