Total and permanganate-oxidizable organic carbon in the corn rooting zone of US Coastal Plain soils as affected by forage radish cover crops and N fertilizer
Introduction
Soil organic matter (SOM) is a key factor of soil quality since it influences nutrient holding and cycling, soil structure, erosion resistance, and soil biological processes (Weil and Magdoff, 2004, Lucas and Weil, 2012). Levels of SOM are most often estimated by measuring total organic carbon (SOC). Because SOC is such a large pool of C and mainly comprised of relatively stable material protected from decomposition, the effects of contrasting soil management practices may take many years to become apparent in SOC measurements (Weil et al., 2003). It remains difficult to measure small quantitative changes in SOC pools caused by variations in soil management practices over short time scales of a few years, despite the fact that these changes may impose significant effects on soil properties and associated microbial processes (Weil et al., 2003). Alternatively, labile soil organic carbon (LOC) is a relatively small fraction of TOC that has a short half life in soils and responds quickly to changes in soil management and fertilization practices (Weil and Magdoff, 2004). The LOC fraction is an important component that determines soil quality because of its involvement in soil aggregate stabilization (Tisdall and Oades, 1982) and its direct link to soil carbon (C) and nitrogen (N) mineralization (Gunapala and Scow, 1998).
Recently Culman et al. (2012), in a meta-analysis of 12 studies, presented evidence that the LOC reactive with a dilute (0.02 M) potassium permanganate solution (Weil et al., 2003) is a microbial processed pool of labile soil C that often exhibits greater sensitivity to changes in management or environmental variation than other commonly measured parameters such as particulate organic carbon (POC), microbial biomass carbon (MBC) or total TOC. They recommended that this fraction be termed permanganate oxidizable carbon (POXC). Several recent studies have reported that POXC was one of the most sensitive and reliable indicators for evaluating the short- and long-term impacts of soil management practices on soil quality (Awale et al., 2013, Chen et al., 2009, DuPont et al., 2010, Melero et al., 2009, Morrow et al., 2016, Plaza-Bonilla et al., 2014, Spargo et al., 2011, Veum et al., 2014). Studies found that POXC quantified by a modified potassium permanganate method (Weil et al., 2003) is sensitive to the changes in SOC content induced by organic amendments (Miles and Brown, 2011), cover crop treatments (Jokela et al., 2009), and high-residue cropping systems (Miles and Brown, 2011). Lucas and Weil (2012) reported that POXC determination is useful for identifying soils where improved SOC management is likely to increase grain productivity and further contribute to soil quality interpretations for producers. Measurement of the POXC content of a soil is also a very simple, inexpensive and non-hazardous method for estimating the LOC fraction (Culman et al., 2012, Morrow et al., 2016, Lucas and Weil, 2012).
Forage radish (Raphanussativus L.) is a unique fall/winter cover crop that is relatively new but becoming rapidly adopted in temperate, humid North America. Forage radish performs a number of unique and desirable functions, including alleviating soil compaction through effective bio-drilling (Chen and Weil, 2010), and efficient capture of N from deep soil layers. The N capture function prevents excess N from leaching into natural waters (Kristensen and Thorup-Kristensen, 2004, Dean and Weil, 2009). The radish has also been reported to increase soil test phosphorous (White and Weil, 2011) and very effectively suppress early spring weeds (Lawley et al., 2011).
Given two months of favorable growing conditions in fall (600+ growing degree days), radish cover crops typically produce 3–8 metric tons/ha of dry matter (approximately 20–30% of which is in the fleshy, partially above ground root). Because of its rapid growth in fall, a forage radish cover crop can add significant quantities of organic carbon to the soil (Mutegi et al., 2011, Mutegi et al., 2013, Dean and Weil, 2009). It is important to keep in mind however that forage radish biomass is highly decomposable so the carbon added to the soil system after radish cover crops has a rapid turnover rate (Kremen and Weil, 2006). More sensitive measures of SOC (e.g., POXC) may be able to detect changes in SOC resulting from radish cover cropping but we could find no published studies to date investigating radish effects on labile soil organic carbon. Moreover, the vast majority of studies measuring cover crop and management effects on SOC have investigated only the upper 10–30 cm of soil, but the few studies that have looked deeper point to the importance of carbon changes in the deep subsoil layers (Baker et al., 2007, Jandl et al., 2014).
This study investigates corn silage production with and without the use of forage radish as a fall/winter cover crop planted immediately after corn is harvested for silage. Within each cover crop treatment (radish or no cover), low and high fertilizer application was also compared. The variables measured included above ground plant C and dry matter production and the distribution of TOC, C:N ratio and POXC in the upper 105 cm of the soil profile. Thus, the objectives of this study were (1) to evaluate the effect of forage radish on soil organic carbon distribution in profile, (2) to determine effects of radish cover cropping on POXC in the soil profile, (3) to measure the effect of band-applied V5 stage corn side-dress nitrogen solution on the soil TOC and POXC in the soil profile, and (4) to determine the relationship between the soil organic carbon and permanganate oxidizable carbon at different soil depths.
Section snippets
Field site description and experimental design
The study was conducted on two fields of the USDA Dairy Farm (39°01′N, 76°89′ W) at Beltsville Agriculture Research Center, Beltsville, Maryland. A completely randomized split-plot design experiment with four replicates was conducted in field BARC1-18 from May 2011 through August 2012 and in field BARC1-21 from May 2012 through August 2013.
The dominant soils types at BARC1-18 are Christiana soils (Fine, kaolinitic, mesic Aquic Hapludults) with silt loam A horizons and clay loam Bt and C
Cover crop dry matter, nitrogen and carbon concentrations and C: N ratio
Forage radish dry matter production ranged from 1965 to more than 3361 kg ha−1 for shoots and 1532 to more than 3740 kg ha−1 for the fleshy tap roots (Table 3). Forage values for radish shoot and root dry matter were significantly higher in the high N than in the low N subplots for any individual site-year. Weeds dry matter was not collected at BARC 1-18 in winter. Forage radish produced less root dry matter than shoot regardless of N level and both radish shoot and root dry matter was increased
Cover crop dry matter, nitrogen and carbon concentrations and C:N ratio
Forage radishes were highly variable in dry matter partitioning, with the fleshy root accounting for 36–55% of total plant dry matter. Dean and Weil (2009) reported similar variability in root/shoot ratio and ascribed this in part to the highly variable size of the fleshy taproot of the radishes, often inversely related to localized plant density. Forage radish displayed the potential to take up large quantities of soil N in fall following silage corn. The N uptake capabilities were especially
Conclusions
There has been very little, if any, previous investigations about the carbon distribution in profile and sequestration potential of autumn-winter established forage radish. Our data indicate that forage radish impacted soil carbon quantities and distribution in surface and deep depth compared with fallow treatments, especially for POXC. Additionally, a strong positive relationship between POXC and SOC has also been displayed in this study. We concluded that although forage radish is grown for a
Acknowledgements
This study was partially funded by the USDA Sustainable Agriculture Research and Education program. We thank Kevin Conover (Univ. of Maryland) for help in taking deep soil cores, Mujen Wang for assistance with field and lab work, the editor and reviewers for constructive feedback.
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