Relationships between crop-species diversity and soil characteristics in southwest Indian agroecosystems

In humid tropical agroecosystems, maintenance of soil fertility is critical to sustainability, yet inputs of fossil-fuel fertilizer are beyond the economic means of many farmers. One economically viable alternative is to manage soil organic matter (SOM) dynamics by altering the composition and number of crop species. The objectives of this study were to determine long-term effects of crop-species composition and richness on indices of soil fertility, in comparison with forests, and to evaluate mechanisms of crop-species influence. Study plots were established in the diverse array of agroecosystems in Kerala, India, where agriculture has been documented for >4000 year. With 91 different crops in the region, analyses were simplified by categorizing crops by functional groups, based on life form, nutrient cycling traits, and phenology. Mono-, bi- and polycultures contained 1, 2 and >2 functional groups, respectively. A paired-plot design in which a monoculture or biculture was paired with a polyculture within a farm was used, such that climate, soil, ownership, and management were as similar as possible for a pair. Soil properties (total soil organic carbon (SOC), N, pH, bulk density) and crop structural attributes that had the potential to influence SOC stocks (basal area and frequency) were measured in 100 m2 plots (n = 70 pairs) within 50 farms. Three plots in nearby forests provided a basis for comparison. Total SOC ranged from 6 to 20 g kg(-1) (in coastal areas and uplands, respectively), compared to 34-60 g kg(-1) in nearby upland forests. Differences in total SOC between mono- and polycultures depended on identity of the functional group in the monoculture: SOC amounts were higher in polycultures if the monoculture in comparison was a palm (P = 0.02) or herbaceous annual (P = 0.001), but not if the monoculture was a deciduous tree. Total SOC was 61% lower in polycultures cultivated for 40-60 years than in nearby forests. Under some conditions, total SOC increased directly with basal area (BA), an index of shading, indicating that BA may be an important design element of agroecosystems. Modeling results indicated that differences in lignin content of crop inputs could influence SOM quantity and quality; hence, crop tissue-chemistry traits may also be important elements of design. These results from ancient, humid tropical agroecosystems suggest that for maintenance of long-term soil fertility, high species richness may not be as important as individual species traits that influence nutrient cycling.