Glomalin-related soil protein responses to elevated CO2 and nitrogen addition in a subtropical forest: Potential consequences for soil carbon accumulation

According to the economy theory, plants should preferentially allocate photosynthate to acquire below-ground resources under elevated atmospheric carbon dioxide (eCO(2)) but decrease below-ground C allocation when nitrogen (N) is sufficient for plant growth. Arbuscular mycorrhizae (AM) represent a critical mechanism of below-ground nutrient acquisition for plants. The dynamics of arbuscular mycorrhizal fungi (AMF) could therefore reflect the response of plant C allocation under eCO(2) and N addition. We examined the responses of glomalin-related soil protein (GRSP) to eCO(2) (approximately 700 mu mol mol(-1) CO2) and/or N addition (100 kg N ha(-1) yr(-1) as NH4NO3) in a modeled subtropical forest to better understand its potential influence on soil C storage. We hypothesized that GRSP would increase under eCO(2) and decrease under N addition. Furthermore, the positive effects of eCO(2) on GRSP would be offset by extra N addition, and GRSP would remain unchanged under combined eCO(2) and N addition. Our results showed that the mean concentrations of easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP) were 0.35 +/- 0.05 and 0.72 +/- 0.13 mg C cm(-3), respectively, which accounted for 2.76 +/- 0.53% and 5.67 +/- 0.92% of soil organic carbon (SOC) in the 0-10 cm soil layer. Elevated CO2 significantly increased T-GRSP by 35.02% but decreased EE-GRSP by 5.09% in the top 10 cm soil layer. The opposite responses of T-GRSP and EE-GRSP to eCO(2) might result from an unchanged photosynthate investment to AMF with possible changes in their decomposition rates. The effect of N on GRSP was contrary to our hypothesis, i.e., there was a 1.72%-48.49% increase in T-GRSP and a slightly increase in EE-GRSP. Both EE-GRSP and T-GRSP concentrations increased under the combination of eCO(2) and N addition, which was inconsistent with our hypothesis. The significant increase of EE-GRSP under the combination of eCO(2) and N addition was partly caused by more rapid plant growth and reduced microbial diversity, and the marginal increase of T-GRSP indicated that the interaction between eCO(2) and N addition offset their independent effects. In addition, the relatively higher accumulation ratios of GRSP (22.6 +/- 13.6%) compared with SOC (15.9 +/- 9.4%) indicated that more rapid GRSP deposition in the soil might accelerate SOC accumulation under eCO(2) and N addition. Our results will improve the understanding of the functioning of GRSP in soil C sequestration under global environmental change scenarios. (C) 2015 Elsevier Ltd. All rights reserved.