In California grasslands, water extractable organic carbon (WEOC) increases as soils dry over the summer. While it is unclear which mechanisms govern this pattern, microbes can tolerate drought stress, suggesting WEOC may increase because exoenzymes continue to break down plant litter without the products diffusing to microbes. To test this hypothesis we manipulated field plots by cutting-off litter inputs and by irrigating and excluding precipitation to extend or shorten the length of the dry season. We hypothesized that the longer the soils would remain dry, the more WEOC would accumulate in the presence of litter, whereas shortening the length of the dry season, or cutting off litter inputs, would reduce WEOC accumulation. We also incubated grass roots in the laboratory and measured the concentration of reducing sugars and potential hydrolytic enzyme activities to understand whether exoenzymes break down litter over the dry season.
Results/Conclusions
As expected, extending the dry season length increased WEOC concentrations by 30% above the 108 µg C g-1 measured in untreated plots, whereas keeping soils moist prevented WEOC from accumulating. Contrary to our hypothesis, excluding plant litter inputs actually increased WEOC concentrations by 40% above the 105 µg C g-1 measured in plots with plants. Reducing sugars did not accumulate in dry senesced roots. Potential rates of reducing sugar production by hydrolytic enzymes ranged from 0.7 to 10 µmol g-1 hr-1 and far exceeded the rates of reducing sugar accumulation (~0.001 µmol g-1 hr-1). Our observations do not support the hypothesis that exoenzymes continue to break down litter to produce WEOC in dry soils. Instead, physical processes are more likely to govern short-term WEOC dynamics through i) slaking of microaggregates that stabilize SOM, ii) WEOC redistribution when soils wet up, and/or iii) through shifts in the orientation of amphiphilic molecules that weaken the strength of bonds between polysaccharides and clays as soils dry.