Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils

Abstract

We used a continuous labeling method of naturally 13C-depleted CO 2 in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean ( Glycine max) and sunflower ( Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO 2 concentration at 400±5 ppm and δ 13C signature at −24.4‰ by regulating the flow of naturally 13C-depleted CO 2 and CO 2-free air into the growth chamber, which allowed us to separate new plant-derived CO 2–C from original soil-derived CO 2–C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO 2–C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO 2–C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO 2–C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity.