Decreases in microbial functional diversity do not result in corresponding changes in decomposition under different moisture conditions

This study compares the functional capability of soils with differing microbial diversity. Soil microbial diversity was modified by either fumigation with reinoculation by unfumigated soil or fumigation with no reinoculation. Functional capability was assessed by following wheat straw decomposition in these soils and in an unfumigated control soil at three matric potentials (−5, −125 and −800 kPa). The changes in diversity after fumigation were compared with the effects of disturbance treatments (slow air-drying, rapid oven-drying, 2 mm sieving and 0.5 mm sieving) by studying patterns of in situ catabolic potential (ISCP) at 1 and 8 weeks. Five weeks after the fumigation treatments, the functional and phenotypic diversity of the soil microbial community, as revealed by patterns of ISCP and phospholipid fatty acid (PLFA) profiles, respectively, were greatly different from that in unfumigated soil. The effects of the fumigation reinoculation treatment on functional diversity were comparable with those caused by rapid oven-drying, but were greater than the effects of 0.5 mm sieving. These disturbance treatments caused persistent changes in functional diversity, whereas slow air-drying and 2 mm sieving had little influence on diversity. Rates of straw decomposition were initially greater in the fumigated reinoculated soil than in the unfumigated soil at all moisture potentials. In contrast, straw mineralisation rates in the fumigated uninoculated soil generally exceeded rates in unfumigated soil for a period after 14 d, which was shorter at greater moisture potentials. These rates resulted in total straw mineralisation in fumigated reinoculated soil exceeding that in unfumigated soil at all moisture potentials. Compared with the unfumigated soils, total straw mineralisation in fumigated uninoculated soil was less at −5 kPa, similar at −125 kPa and greater at −800 kPa. The results indicated that the decomposition function of soil with reduced functional diversity can be diminished under optimum moisture conditions, but is not invariably reduced when assessed under suboptimal moisture conditions. This indicated that decreases in the functional diversity of soil microbial communities may not consistently result in declines in soil functioning.