Bacterial Growth in Milpa Polyculture and Monoculture Soils [Emory University]

Abstract

Polycultures, or multicrops, are groupings of plants that grow more prolifically when planted together as compared to when planted alone as monocultures. One of the best known and widely utilized polycultures is the milpa cropping system - the cultivation of maize, beans, and squash together as “the three sisters.” Milpa has been utilized by the indigenous population of Central America for millennia due to its consistent abundant harvests; today it remains a cornerstone of the region’s tradition, diet, and economic growth. Likely contributing to this legacy is the known association of polycultures and heightened resistance to disease, yet the mechanism underlying this relationship in milpa has largely been unexplored. To assess the health of farm soil exposed to milpa monocrops, bicrops, and multicrops, we measured the growth of two bacterial strains: a Burkholderia strain symbiotic of Anasa tristis (the squash bug, an agricultural pest) and a plant pathogenic Serratia strain that is the primary causal agent of cucurbit yellow vine disease (CYVD). We found that after one week in both the polyculture (corn, bean, and squash) soil and the corn monoculture soil, the growth of Burkholderia was significantly inhibited. However, in both corn & bean and bean & squash biculture soils, the growth of the strain was significantly enhanced. The growth of the Serratia strain did not yield any significant increase or decrease after one week in any milpa soil. We conclude that the cultivation of milpa in its polyculture configuration demonstrates antibiotic activity towards the Burkholderia strain SQ4A. Our investigation supports findings that certain multicrop systems are less susceptible to disease than monocultures possibly due to their greater microbial biomass; thus we can infer a higher amount of root exudates present in the soil, of which a substantial amount may be anti-microbials.