Understanding biogeochemical C and N cycling belowground in different ecosystems is important to predict C sequestration and trace gas fluxes related to global climate change. Warm-season C₄ grasses are highly responsive to colonization by arbuscular mycorrhizal fungi (AM fungi). This symbiosis not only provides a direct link between the plant root and the surrounding rhizosphere bacteria and fungi, but also supports soil fauna including nematodes. The interrelationships between these biological components are possible drivers of C and N turnover throughout the ecosystem. The objective of this experiment was to (1) evaluate changes to belowground C pools and microbial community structure, (2) evaluate the importance of AM fungi root colonization on the cycling of C and N, (3) measure the responses of plant and microbial grazing nematodes to the presence or absence of AM fungi in different ecosystems (Figure 1). A field experiment was initiated in April 2004 in three different C₄ grass ecosystems: tallgrass prairie species dominated by big bluestem (Andropogon gerardii) and indiangrass (Sorghastrum nutans) (NP); no-tillage grain sorghum (Sorghum bicolor) (NT); and conventional tillage grain sorghum (CT). The field was previously cropped to C₃  wheat (Triticum aestivum) for 8 out of the past 9 y and the soil C had a d ¹³C signature (relative to PDB) of -19.11‰, and thus was suited for observing C₄-C inputs over time into C₃-C soil organic pools. Initial soil phosphorus (P) levels were 8 mg Olsen P/kg soil. Additions of P (total 90 mg P/kg soil) were made to the soil surface as a split plot to suppress AM fungi colonization of plant roots. Plots were sampled in October 2004, April 2005, and November 2005 for soil total C and N, microbial biomass C and N (MBC and MBN, respectively), phospholipid fatty acids, and nematode diversity and abundance. Plants were sampled for aboveground and belowground biomass, total C, N, and percent root colonization by AM fungi. Results from October 2004 indicate that MBC was not different between treatments with a value of 282 mg C/kg soil. The addition of P increased MBN in NP, but decreased MBN in NT and CT, with values of 16.4, 21.0, 17.4, 15.4, 22.1, and 13.9 mg MBN/kg soil for NT-P, NT+P, NT-P, NT+P, CT-P, and CT+P, respectively. The P addition resulted in lower, similar, and higher AM colonization in NT, NP, and CT, respectively. Herbivorous nematodes were generally twice as numerous in NT and CT as in NP, while microbial grazing nematodes were twice as numerous in NP as in NT or CT. We were also able to detect ¹³C signature differences in nematodes, with some types having a transitional C₄ to C₃  signal, indicating a more direct role in C cycling; others, however, were shown to show a C₃ signal, indicating a more indirect role in C cycling. Preliminary results in the ecosystems evaluated are providing interesting information indicating that C and N processing is ecosystem specific and that belowground biology may be as important as plant species in carbon cycling.

Figure 1. AM fungi, nematodes, and microbes all play an important role in belowground C, N, and P cycling which impacts global climate change.