Organic mulches and high residue no-till for collard production in Alabama

Weed suppression.

Mulching provided weed suppression of broadleaves, grasses and sedges, but forage soybean as a summer cover crop did not (Table 1), likely due to the fact that soybean residue decomposes too quickly to have a lasting mulching effect. In all cases, days after mulching (DAM) were highly significant within each year of the study (P < 0.0001). A time by mulch interaction within each year (DAM*Mulch(Year)) was due to both the effect of mulch application and the growth of weeds as the season progressed. Evidence of a mulch by year interaction suggested that weed populations were affected by mulching for three consecutive years. This effect was most apparent on broadleaf control (Figure 1). Mulching the first year was effective for suppression of broadleaf weeds. Suppression of broadleaf weeds during the first year appears to lessen broadleaf infestation during subsequent years, although mulching during 2007-8 did not have the same level of suppression compared to the non-mulched control. Since conservation tillage tends to shift weed populations away from broadleaves (Teasdale et al., 1991; El Titi, 2003), it is not surprising that broadleaf control can be enhanced with mulch application.

The population shift toward grasses under conservation tillage makes grass control more difficult (El Titi, 2003). During the first year of no-till, grass control was not effective using any of the mulches (Figure 2), but subsequent years showed much better grass suppression using mulches compared to the non-mulched control. Grass infestation was maintained below 10% by the application of any mulching material in 2007 (compared to 17% for the non-mulched control), and below 6% in 2008. The data show that mulching suppresses monocot weed populations in no-till systems if used for more than one year compared to the control, and may suggest that >2 years of no-till with high-biomass producing cover crops may be effective at reducing grassy weeds, although more data are needed to support this claim. In any case, the data show that grass populations under no-till are highly variable, with populations increasing dramatically during the second year of conversion from conventional tillage, but decreasing in the third year. Mowing grasses before seed heads become viable may reduce the grass populations to manageable levels during transition to conservation tillage.

During the first year of transition from conventional to conservation tillage, yellow nutsedge (Cyperus esculentus L.) control was highly problematic, with total coverage ranging from 7-21% (Figure 3). However, subsequent years of high residue no-till improved sedge suppression, generally below 5% coverage, although there was not much difference between any of the mulching treatments and the control. Bangarwa, et al. (2008) showed that straw mulch applied at 7300 kg/ha (7 cm thick) was effective at reducing medium (0.26-0.50 g) purple nutsedge (Cyperus rotundus L.) tuber density, but did not reduce large (>0.50 g) or small (0.10-0.25 g) tuber density for bell pepper production in Clemson, SC. They also found generally comparable tuber density when tilled plots were either straw-mulched at transplanting or hand-weeded every 1-2 weeks. It is interesting to note that sedge coverage was subject to a significant cover crop by mulch interaction (Table 1), resulting from increased sedge suppression by mimosa prunings after a forage soybean summer cover crop in 2006 and increased sedge suppression achieved in control plots when combined with forage soybean in 2007 and 2008 (Figure 3). Although it is unclear why this should be the case, it is apparent that sedge suppression is improved during subsequent years of high-biomass producing cover crops in combination with no-till, with or without the application of mulches. Yellow nutsedge was the only perennial weed species present after three years (Fig. 4).

Figure 4 shows the weed infestation 6 d before and 15 d after mulching in 2008 by species averaged over all plots, giving an overall picture of the weeds present three years after initiation of no-till. No individual weed species covers more than 4% of the surface 15 days after mulching, or five weeks after transplanting. This suppression should give the main crop an adequate start to compete successfully with weeds later in the season. Yellow nutsedge was the single major species present at that time, followed by large crabgrass. After three years of high-biomass no-till, the only grasses present were large crabgrass and winter rye, the latter due to viable seed germination from the previous winter cover crop. This underscores the importance of ensuring termination of cover crops and mulches before seeds become viable. Summer annual broadleaves consisted of spiny pigweed (Amaranthus spinosus L.), common purslane (Portulaca oleracea L.), carpetweed (Mollugo verticillata L.) and cutleaf groundcherry (Physalis angulata L.), though all of these weeds were under very good control three years after initiation of no-till. It may be that these were not a major problem because experiment occurred in the fall or because of residual allelopathy from the winter rye. Burgos and Talbert (1996) found that rye, wheat, and rye with hairy vetch suppressed 70-85% of redroot pigweed (Amaranthus retroflexus L.) and yellow nutsedge eight weeks after cover crop termination without herbicides, and that rye alone and rye with vetch suppressed 65-70% of large crabgrass (Digitaria sanguinalis (L.) Scop.).

Among the winter annual broadleaf weeds, wild radish (Raphanus raphanistrum L.) exhibited much greater average groundcover than henbit (Lamium amplexicaule L.), though the average coverage was still less than 2%. Even so, fall mulching was not effective for wild radish control 15 d after application (P=0.6738) in 2008 (Figure 5). The same can be said for all the major weed species present 15 days after mulching during 2008 with the exception of large crabgrass. This may be due to the fact that weed coverage was already under considerably good control after three consecutive years of no-till with high-biomass cover crops, given that even non-mulched control plots exhibited less than 4% coverage of any particular species. The mat of residue on the soil surface after three years of no-till appears to be effective at weed suppression. Figure 5 also shows that while not statistically significant, straw mulch tends to be the best suppressor of the major weed species in 2008, likely due to the thickness of the straw residue.

Collard yield was not significantly different for any variable, including year (data not shown). This is not unexpected since crop yields are generally more responsive to tillage systems and management than to weed density alone (El Titi, 2003).

Persistence and decomposition rates.

Decomposition of organic residue occurs in two phases. Initially, a labile portion of the residue, such as sugars, starches and proteins, is readily consumed by soil microbes, leaving behind a recalcitrant portion of the residue, such as cellulose, fats, waxes, lignin and tannins (Wieder and Lang, 1982). This recalcitrant portion is slowly decomposed and contributes to the development of organic matter in soil. Such a system is best described by double exponential decay models, with one exponential segment describing the labile portion and the other exponential segment describing the recalcitrant portion of the residue (Wieder and Lang, 1982). The double exponential decay model is represented by the equation Y = Ae^(-k1*t) + Be^(-k2*t), where Y = the nutrient or mass remaining, A = the labile portion, B = the recalcitrant portion, k1 and k2 are rate constants fitted to the data, and t = time in days after application. This model serves as the basis for comparison of N, C, and mass loss between conservation and conventional tillage in this study.

Mass loss from organic residues under conservation and conventional tillage is shown in Figure 6. Buried residue generally exhibits faster mass loss in both the labile and recalcitrant portions of all residues, as shown by the greater rate constants k1 and k2 for buried material compared to surface residue (Table 2).

Nitrogen loss from organic residues under conservation and conventional tillage is shown in Figure 7. Buried residue generally exhibits faster N loss in both the labile and recalcitrant portions of all residues. This is evidenced by the greater rate constants k1 and k2 for buried material compared to surface residue (Table 2), though notable rate constant exceptions exist in cases where the curve fit (Adj. R^2) is exceptionally low, such as in the case of straw, which has a very low original N content and negligible labile N pool. For residues with a high N content, there is considerably more N potentially available to a spring crop from surface residue than buried residue. For example, at planting on May 1 (day 204), there is approximately 33 lbs/ac more N potentially available from surface soybean residue than incorporated soybean residue. Upon mineralization, N is subject to the competing processes of nitrification, immobilization, plant uptake, ammonium fixation, and volatilization. This study does not determine the fate of the lost N (i.e., the proportion mineralized, immobilized, etc.).

Carbon loss from organic residues under conservation and conventional tillage is shown in Figure 8. Buried C loss models appear similar to buried mass loss models (Figure 6) because most mass loss is due to the respiration of C, which is then lost to the environment as CO2. Buried residue exhibits faster C loss in both the labile and recalcitrant portions of all residues, as shown by the greater rate constants k1 and k2 for buried material compared to surface residue (Table 2). Carbon is therefore sequestered longer when residue is left on the surface compared to residue incorporation. This should result in greater SOM accumulation from surface residue over time. On a more speculative note, in an age when producers may be compelled to participate in a C market, conservation tillage practices may provide producers with a C offset or credit, while also enhancing SOM.