Determination of Decomposition Rates of Cover Crop Residues and Their Nutrient Release Characteristics

Table 3: Spring cover crop data.  Letters represent least significant differences within location, from an analysis including five cover crop treatments.  Only pennycress and rye are included here.

Table 4: Average nutrient content in cover crop biomass in mg/ha at time of termination.  Nutrient content was analyzed from homogenized bulk samples, and total cover crop biomass was estimated in individual plots.

Table 5: Plant tissue composition at termination and burial.  Letters represent Least Significant Differences within the response variable.  Two outliers were removed from nitrogen analysis.

Table 6: Cell wall composition of plant tissue at time of termination. Letters represent Least Significant Differences within each column.

Details of cover crop composition in the field at termination are given in Table 1 and Table 2.  The C:N ratio was higher for rye than for pennycress, but both are below 20, indicating that net nitrogen mineralization from decomposition should occur.  In Waseca, pennycress had a greater composition of sulfur, which is expected given that pennycress is in the brassica family.  Although we did not test for glucosinolates, it is possible that higher levels of these chemicals in pennycress tissue could slow decomposition given their toxicity to soil microbes.

We tested plant tissue for nitrogen and carbon content at termination and burial to determine if any nitrogen is being lost during this period (see Table 5).  Small differences in C:N ratio seem to be driven by a loss of carbon rather than nitrogen.  These results don't suggest that significant nitrogen volatilization occurred, although biomass dried in the lab may not perfectly reflect what is occurring to biomass dried in the field.

Hemicellulose and lignin content of cell walls have been shown to impact decomposition rates.  As lignin is the most recalcitrant fraction, higher levels in pennycress suggest this species will decay more slowly (see Table 6). However, higher levels of hemicellulose (which is also fairly difficult to decompose) were found in rye.  The differences in cell wall composition between cover crop species were consistent across both locations.  Because of our sampling method, we were not able to include these variables as covariates in our analysis of decomposition.

Table 7: Statistical significance of models for decomposition (percent remaining over time) of plant biomass and nutrient content for first three removal times. 

Loc=Location; Y=response variable;  Significance: *=<.1, **=<0.05,***=<0.01,****=<0.001; Corr=Residual correlation structure: 1=none, 2=1^st order autoregressive, 3=compound symmetry, 4=unstructured.

Table 8: Estimates of coefficients and their standard errors for decomposition curves of the form: Y~a+b*e^c*days since burial. Only models that were able to fit a curve are included.

 Significance: *=<.1, **=<0.05,***=<0.01,****=<0.001.

Figure 1: Decomposition curves for decomposition of biomass (as dry matter).  Details of the fitted curves are given in Table 7.

There was sufficient plant material for nutrient analysis by plot only for the first three of six removal times.  The effect of cover crop and nitrogen rate on decomposition of total biomass and nutrient release over this time period was tested for each location.  (There was also insufficient biomass to analyze nitrogen in three plots from Waseca in the third removal time, so the effect nitrogen rate could not be analyzed for nitrogen decomposition and C:N.)  Details of treatment significance for the release of each nutrient and total biomass are given in Table 7.  Elevated levels of metals (compared to unburied plant material) likely result from incomplete removal of soil from the plant material prior to analysis.  This is difficult if not impossible to avoid with a mesh litter bag methodology.  Decomposition curves were not able to be fitted to most nutrients, likely as a result of this soil contamination.  However, cover crop tissue content for these nutrients was not great enough to cause a deficiency in corn (see Table 4), so the timing of their release from decomposing plant material is less relevant to growers than the release of macronutrients nitrogen, phosphorous, and potassium (sulfur was not analyzed).

Details of decomposition curves are given in Table 8.  The first coefficient, "a", represents the value of Y (percent remaining) at which the curve plateaus, so a higher value of "a" indicates incomplete decomposition during the growing season.  The "b" coefficient is simply (100-a).  The third coefficient, "c", is the natural log of the decomposition constant.  The more negative the value of "c", the faster the rate of decomposition.  Figures of the plotted decomposition curves are given for biomass decomposition (see Figure 1).  Fertilizer nitrogen rate was only significant in Waseca, and there was no interaction between cover crop and nitrogen rate.  The estimates of plateau values for both nitrogen rates were not significantly different from zero, indicating complete decomposition.  The decomposition constants indicate a slightly faster decomposition where nitrogen fertilizer was applied, which may come from a "priming effect" where extra nitrogen spurs microbial activity in the soil.  Decomposition of plant biomass differed between cover crops in both locations.  Although decomposition was more rapid in pennycress tissue, the plateau values are higher for pennycress.  Importantly, however, across treatments and locations at least 70 percent of plant tissue had decomposed within a month of burial (ie, by the time period when corn nitrogen demand is highest).  Biomass decomposition was the dominant driver of nutrient release, so it is not surprising that nitrogen and phosphorus release was also rapid.  Potassium, which is present in plant tissue as a water-soluble ion was released even more quickly, as expected.  

Table 9: Critical values (in lb N/ac) from corn grain yield response curves for three cover crop treatments in 2017.

In agronomic research, environmental variability means that results often vary between years, so we must be cautious generalizing from a single year's data.  We can conclude that at least under certain conditions, rye and pennycress cover crops will decompose and mineralize nitrogen, phosphorous, and potassium quickly enough that corn deficiencies are unlikely.  Nevertheless, corn grain yields from the larger study indicate that greater nitrogen fertilizer applications may be necessary to maximize corn yields following these cover crops compared to no cover crop (See Table 9).  In this year, maximum yield potential was similar among cover crop treatments in Waseca but lower following cover crops in Rosemount compared to the control.  Given the rapid decomposition of cover crop biomass and mineralization of nitrogen, the impact on corn yield from cover crops may be caused by some factor other than nitrogen cycling.  Further studies of this kind will aid in our understanding of the impact of cover crops on nutrient cycling, and will allow better recommendations to growers interested in incorporating cover crops in their corn-soybean rotations.