Plant functional composition influences rates of soil carbon and nitrogen accumulation

Abstract

Summary 1.The mechanisms controlling soil carbon (C) and nitrogen (N) accumulation are crucial for explaining why soils are major terrestrial C sinks. Such mechanisms have been mainly addressed by imposing short-term, step-changes in CO 2 , temperature and N fertilization rates on either monocultures or low-diversity plant assemblages. No studies have addressed the long-term effects of plant functional diversity (i.e. plant functional composition) on rates of soil C accumulation in N-limited grasslands where fixation is the main source of N for plants. 2.Here we measure net soil C and N accumulation to 1 m soil-depth during a 12-year-long grassland biodiversity experiment established on agriculturally degraded soils at Cedar Creek, Minnesota, USA. 3. We show that high-diversity mixtures of perennial grassland plant species stored 500% and 600% more soil C and N than, on average, did monoculture plots of the same species. Moreover, the presence of C4 grasses and legumes increased soil C accumulation by 193% and 522%, respectively. Higher soil C and N accrual resulted both from increased C and N inputs via (i) higher root biomass, and (ii) from greater root biomass accumulation to 60 cm soil depth resulting from the presence of highly complementary functional groups (i.e. C4 grasses and legumes). 4. Our results suggest that the joint presence of C4 grass and legume species is a key cause of greater soil C and N accumulation in both higher and lower diversity plant assemblages. This is because legumes have unique access to N, and C4 grasses take up and use N efficiently, increasing below-ground biomass and thus soil C and N inputs. 5. Synthesis. We demonstrate that plant functional complementarity is a key reason why higher plant diversity leads to greater soil C and N accumulation on agriculturally degraded soils. We suggest the combination of key C4 grass-legume species may greatly increase ecosystem services such as soil C accumulation and biomass (biofuel) production in both high-and low-diversity N-limited grassland systems.

Interactive Effects of Time, CO2, N, and Diversity on Total Belowground Carbon Allocation and Ecosystem Carbon Storage in a Grassland Community

“…They are also consistent with experiments finding elevated CO 2 to increase soil C turnover rates, resulting in little or no change in soil C (Hungate and others 1997;Van Kessel and others 2000;Hagedorn and others 2003;Lichter and others 2005). Our results contrast with those of Fornara and Tilman (2008) who found diversity and N addition to increase ecosystem C storage via increases in total soil C in an annually burned grassland. In BioCON, soil C did not change significantly over time (see Methods).…”

“…In BioCON, soil C did not change significantly over time (see Methods). These contrasting temporal soil C patterns may be due to the relatively C depleted initial state of the soils in Fornara and Tilman's (2008) experiment (approximately 0.48% versus an average of 0.63% at the beginning of BioCON and 0.60% in the diverse plots at the end of the Fornara and Tilman (2008) experiment).…”

“…These estimates may be high, however, as rates of C accumulation in litter may be at a maximum in years following burns and saturate after multiple years without burning, because the potential for C litter storage in grasslands is low (relative to woody systems). Increased litter inputs in unburned grasslands could translate into increased soil C storage as litter is decomposed and incorporated into soil organic matter (Fornara and Tilman 2008;McLauchlan and others 2006), and perhaps especially so under elevated CO 2 . However, unburned grasslands may ultimately undergo conversion to savannah or woodland, thereby further increasing C sequestration in biomass and potentially in soils (Tilman and others 2000;Hibbard and others 2001;McKinley and Blair 2008).…”

Plant diversity increases soil microbial activity and soil carbon storage

“…This was partly unexpected as legumes have been reported to increase the availability of nitrogen in soils 35,36 and are therefore suggested to increase plant biomass production (above and below ground) 24 , resulting in increased metabolic activity of soil microorganisms 37 . Figure 2 | Conceptual model on the expected causal relationships between plant diversity and soil carbon storage.…”

“…Alternatively or in parallel, higher amounts of plant residue inputs could enhance carbon storage through increased microbial necromass accumulation over time 23 . Therefore, although soil carbon storage increased with plant diversity in a number of long-term grassland experiments 24,25 , the underlying mechanisms are yet to be understood.…”