Soil carbon (C) accumulation in agricultural landscapes can improve soil health and concurrently mitigate climate change. My dissertation addresses three major knowledge gaps with respect to root production and soil C accumulation within agricultural landscapes: Nitrogen fertilizer additions, life history (annual versus perennial), and biodiversity. In addition, I investigate how farmers perceive soil C on their fields and determine which soil C indicators best reflect their perceptions of soil health. Planting perennial grain crops in place of annual row crops could lead to C sequestration due to their extensive root systems. In chapters 2 and 3, I test the optimal partitioning theory and examine soil C cycling of annual winter wheat (Triticum aestivum) and perennial intermediate wheatgrass (Thinopyrum intermidum; IWG) under three nitrogen levels (Low N (Organic N), Mid N, High N). I found that IWG had significantly greater root biomass at surface depths compared to wheat (P<0.05), but there were no differences at subsurface depths between the two crops. In 2011 and 2012, total root biomass remained stable across the three N levels for both crops but in 2013, IWG root biomass in the High N level was significantly greater than in the Low N (Organic N) and Mid N levels (p<0.05). Despite significantly greater root C in IWG, there were no differences in labile or recalcitrant C pools compared to wheat. Overall, these results fail to support the optimal portioning theory and findings suggest that a longer period of time is needed in order for soil C to accumulate under perennial grain crops. The ability to sequester C could be a major benefit of perennial cellulosic biofuels. In chapters 4 and 5, I examine fine root production and soil C dynamics via a long-term incubation in candidate biofuel cropping systems that differ in life histories (annual vs. perennial) and diversity (monoculture vs. polyculture) in contrasting soils. I found that the native grasses and restored prairie systems had greater root production compared to the monoculture perennials (p<0.05). At the low fertility site, I found substantial differences in active C pools between annual and perennial polyculture crops. Active C pools under polycultures were over 2.5 times greater than under continuous corn. At the high fertility site, most system differences were insignificant except the restored prairie and rotational corn had 3.4 times more active C than other systems. I conclude that diverse perennial biofuel crops grown on marginal lands are more effective at C accumulation compared to diverse perennials grown on high fertility soils. In chapter 6, I compare the total soil organic matter test to the C mineralization (active C) test to determine which soil C indicator reflected differences in management across 52 farm fields in Michigan and whether test results reflect farmer perceptions of soil C. Results from the active C test strongly supported investigator field observations and farmer perceptions of soil C. My findings demonstrate that the active C test should be widely offered at university and commercial laboratories. Overall, these results show that roots of established perennial grain crops increase with greater N additions, which can lead to large C stores and N retention in roots. However, in two separate experiments, I found no evidence for enhanced soil C accumulation over the first 4-5 years under monoculture perennial cropping systems relative to annual row-crops. This suggests that crop diversity in perennial based cropping systems should be promoted to replenish soil C for increased soil health and climate change mitigation.
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