Soil Microbial Communities and Activities Under Different Orchard Floor Management Systems in Oregon Sweet Cherry Orchards.

Although the importance of microorganisms in nutrient cycling and productivity is well recognized in annual cropping systems, specific information regarding the size, composition and activity of soil microbes in orchard systems is lacking. This study assessed the soil microbial community structure (SMCS) and enzyme activities involved in C, N, P, and S cycling in soil samples (0-15 and 15-30cm) under different orchard floor management systems in two organically managed and two conventionally managed sweet cherry orchards. We compared the use of organic amendments to landscape cloth within the tree rows of the organically managed orchards. Within the conventional orchards, we compared the use of straw mulch to a herbicide strip. Although soil organic carbon (SOC) did not increase at the organic site where straw mulch was the amendment, we found a shift in SMCS with greater fungal and gram negative bacterial markers and fungal:bacterial ratios under straw mulch compared to cloth. Straw mulch treatments also had greater P- and S-cycling enzyme activities at 0-15cm depth and greater N- and S- and P-cycling enzyme activities at 15-30cm depth. In contrast, SOC increased up to 16% at the second organic site where two applications of bark mulch and one application of municipal leaf compost had been applied. Greater bacterial biomarkers were found under cloth. Fungal biomarkers were not increased under mulch as expected with the applications of bark mulch. Only the N-cycling enzyme was sensitive to treatments at this site with increases up to 75% under the mulch treatments compared to cloth. At the conventional orchards comparing straw mulch to the herbicide strip, no changes in SOC was found between treatments. However, mulch treatment increased enzymatic activities between 31 and 93% for enzymes involved in C, N, and S cycles. Our results suggest that changes in the soil microbial community and their activities are sensitive measures of management in orchard systems. These biological indicators are directly involved in nutrient cycling and C storage potential and should be considered as another tool for assessment of overall orchard health.