Do dead plants control C cycling in dead drought-stricken soil?

Background/Question/Methods

It would be wrong to assume that carbon (C) cycling stops in dry soil. After plants die during the dry season or drought, soil microbes survive in microscopic patches of remaining water, thus potentially influencing soil C processing long after direct plant influences end. And even without live microbes, extracellular enzymes and purely abiotic processes may continue to decompose soil C. Our goal was to quantify the legacy of dead plants on soil C pools during the dry season and in the absence of live microbes. In a semiarid grassland in southern California, we created a gradient of plant C input by hand-weeding 2 m² plots during the winter growing season. After plant senescence, we created a gradient of dry season length in full-factorial design by shortening with irrigation or lengthening with rainout shelters. Soils were collected bimonthly to track changes in microbial biomass (MB), water-extractable organic C (WEOC), and respiration 2 h (“short term”) and 6 months (“long term”) following rewetting. To quantify extracellular processes, we killed soil MB in the laboratory with continuous chloroform fumigation for 6 months while monitoring carbon dioxide (CO₂) production.

Results/Conclusions

During the dry season, WEOC increased by 69% when plants were present and by 149% when plants were absent. This result was opposite of our expectation that plants would promote WEOC accumulation during the dry season. Greater accumulation of WEOC in the absence of plants, regardless of dry season length, suggests that plants somehow inhibited decomposition. Alternatively, because dead plants supported greater MB during the dry season, perhaps these extra microbes ate more of the solubilized C. Shortening the dry season by 2.5 months decreased WEOC and short-term respiration by 37—61% but only decreased the C pool for long-term respiration by 1%. Plants, however, increased long-term respiration by 31%. In the sterile soil incubations, plants did not increase CO₂ production suggesting that the underlying decarboxylation reactions are targeting older C. Independent of plants, the contribution of extracellular CO₂ production to the total live CO₂ flux was 2—5% in moist soil and 72—90% in dry soil. In conclusion, dead plants do appear to influence soil C cycling during drought because of the actions of live microbes.