Improvements in soil health and soil carbon sequestration by an agroforestry for food production system

Abstract

Woody perennial cropping systems have the potential to produce food while maintaining or improving soil health relative to annual cropping systems. We used space-for-time substitution to assess soil health improvements by a Chinese chestnut – pawpaw agroforestry (AF) system in Iowa, USA relative to corn – soybean rotation (CSR), representing conventional annual crop management, and secondary forest (SF), which served as a natural woody perennial ecosystem reference point for improved soil health. We also compared several Chinese chestnut (Castanea mollisima Blume) and pawpaw (Asima trilobal (L.) Dunal) AF guilds, including single and double density chestnut monoculture, pawpaw monoculture, and chestnut and pawpaw polyculture, to evaluate the effects of tree density versus diversity on soil health. We found significant improvements in biogeochemical metrics of soil health for a mature (24 years old) AF system established on fields previously in annual crop production, relative to an adjacent no-till CSR system. Surface soil (0–30 cm depth) total and microbial stocks of carbon and nitrogen in the agroforestry system were greater in AF than in CSR, but less than in the SF. Soil carbon stocks down to one meter increased from CSR (62.7 Mg C ha-1 ± 3.22 SE) to AF (72.1 Mg C ha-1 ± 2.23 SE) and SF systems (80.8 Mg C ha-1 ± 4.46 SE), but did not differ between the two perennial systems. Acid - base chemistry in surface soils was similar between AF and CSR, but SF ecosystems had significantly higher pH, CECe, and percent base saturation than both cropping systems. We found few effects of AF guild (i.e. groupings of woody crop species) on the biogeochemical metrics of soil health, suggesting that modest increases in woody perennial crop density and diversity did not lead to further improvements to soil health. Overall, land conversion to agroforestry from annual crop production improved soil health and increased soil carbon sequestration.