Carbon input by mycorrhizas into soil regulated by nitrogen fertilization

The input, distribution and fate of photosynthesized carbon (C) in plant-soil systems is essential for understanding the terrestrial C cycle. It is known that a significant fraction of C stored in the soil moves through arbuscular mycorrhizal fungi (AMF). However, the impacts of AMF on plant C input and soil C budget are poorly understood. A mycorrhiza defective mutant of tomato (rmc) and its mycorrhizal wild type progenitor (76R) were used to control for the formation of AMF symbiosis. Continuous ¹³CO₂ labeling was performed to quantify the photosynthetic C allocation within plant-soil system depending on AMF symbiosis and nitrogen (N) fertilization. AMF symbiosis decreased relative C incorporation (% of total assimilated C)into roots, but the net rhizodeposition remaining in the SOC pools for 76R plants was greater than that for rmc plants (P<0.05). Further, cumulative rhizosphere respiration did not differ between two tomato genotypes, however, respiration of mycorrhizal roots per unit dry weight was higher than rmc because of fungal association with roots. N fertilized decreased relative C incorporation into root, rhizosphere soil, and bulk soil. At 16 weeks of labeling, estimated amount of C transferred remaining in rhizosphere and bulk soil by 76R was 0.02-0.04, 1.67-1.75 mg C kg^-1, of which rhizodeposition accounted for 0.7-1.1% and 42-46% of C remaining belowground, respectively. In the rmc plants, however, the rhizodeposition remaining in rhizosphere and bulk soil was around 0.02, and 1.34mg C kg^-1, accounting for maximum 0.6% and 34% of C remaining belowground, respectively. We concluded that the larger amount of rhizodeposition incorporated into SOC pools for 76R compared with rmc plants, was induced by AMF symbiosis, and the smaller difference between 76R and rmc in fertilized soil than in unfertilized soil, suggesting that N fertilization reduced the C flow from plants to soil as well as the AMF abundance. Meanwhile, N fertilization raised the belowground input mainly by rhizodeposition, and decreased SOM mineralization, and consequently reduced net C losses from fertilized soil.