We evaluated three cover crops (winter rye + hairy vetch, oat + field pea, and medium red clover) in zone tilled organic yellow crookneck squash production for biomass and N content at early and late termination dates. We also measured soil quality in crop rows and between rows using potentially mineralizable N (PMN), soil inorganic N, and permanganate oxidizable C (POXC). Squash yield did not vary by cover crop treatments. Cover crops left between crop rows to fully mature did gain biomass and N content. In 2016, PMN detected in-row mineralization of winter rye + hairy vetch residues at the mid-season sampling when inorganic N increased. Interestingly, POXC was affected only slightly by treatments, likely due to the assay’s sensitivity to soil organic matter stabilization rather than nutrient cycling. Results echo the role that residue C/N plays in soil N cycling. Moreover, the extended growing period for cover crops between crop rows in the first several weeks of crop growth appeared to boost soil fertility while having no negative effect on crop yield.
Soil quality is described by Doran & Parkin (1994) as the degree to which soil functions within an ecosystem to sustain biological productivity and plant and animal health, while maintaining long-term environmental benefits. Improving soil quality and biological activity is important in organic cropping systems where crop nutrition is provided via transformation of organic amendments into plant available, inorganic nutrients by soil microbes. As such, the National Organic Program strongly encourages the use of cover crops, diversified crop rotations, and reduced tillage practices (Agriculture, 2007). Soil quality is achieved in part through continued ground cover, often accomplished with the use of cover crops and by leaving cover crop residue on the soil surface. In turn, the use of zone tillage has been proposed as a way to incorporate cover crop residue only where crops will be planted and to maintain living cover crops between rows to prevent N mineralization where it is liable to leaching (Brainard et al., 2013; Williams et al., 2016). This approach has great potential in Northern climates ,where winter annual legumes rarely reach maturity by vegetable planting, which makes it difficult for growers to gain as much N from legumes as is fully possible (Zhang et al., 2013). Maintaining ground cover between rows with these cover crops until flowering may increase N input and other ecosystem services.
The goal of our study was to investigate the effect of zone tillage, allowing for extended cover crop growth in vegetable systems, on cover crop N contributions, soil quality, and yield of certified organic yellow crookneck squash (Cucurbita pepo). Specifically, our objectives were to 1) assess the effect of extended cover crop growth on organic matter and N contributions, 2) compare the effect of zone till management on in-row soils where immature cover crop residue was incorporated via tillage to between-row soils where soil is undisturbed and roots are present, and 3) determine how squash yields were affected by cover crop contributions to soil quality.
This experiment took place in St. Paul, Minnesota under certified organic production. In 2015, a randomized complete block design with four blocks was used with split plot treatments, where cover crops (oat/pea mix, OP; winter rye/hairy vetch, RV) were whole plots and tillage (full-width conventional, CT; strip-tilled, ST) was the sub plot factor. In 2016, a randomized complete block design was used with only three factors (OP+CT, RV+ST, and red clover, C+ST) and four blocks. In the second year, oat and pea was used only for CT plots since both species winter-kill in Minnesota. A red clover treatment (C) was added due to its biennial potential and low stature.
Cover crop management
For the first year, plots were flail mowed and rototilled in spring once weather permitted, and cover crops were seeded approximately one week later. For the second year, RV and C were fall planted and OP was spring planted. In both years, winter rye (Secale cereale) was seeded at 56 kg ha-1 with hairy vetch (Vicia villosa) at 22.4 kg ha-1. Oat (Avena sativa) and field pea (Pisum sativum) were seeded at 56 kg ha-1 in both years. Medium red clover (Trifolium pretense L.), planted only for the second year, was seeded at 13.5 kg ha-1.
Cover crops were terminated at early (before crop planting; in row areas and between rows for C), and late (living cover crops between rows in ST plots at legume maturity) time points. In 2016, hairy vetch pod set matched the early termination time, so both between- and in-row areas were terminated. A flail mower and rototiller was used at the early time point, and only a flail mower at the late termination to keep residues on the soil surface between rows. In both years, weed seed heads were clipped and removed in order to reduce weed pressures in following years.
Cover crop sampling
Cover crops were sampled twice each year immediately before each termination using two combined cuttings from 0.1 m2 quadrats. Species in cover crop mixes, as well as weeds, were separated. Sampled were dried at 60ºC for at least 48 hours, then were ground to 1 mm and analyzed for C and N content on a combustion analyzer (Elementar VarioMAX CN analyzer, Elementar Americas).
Soils were sampled from in- and between-row areas four times per year: 1) before spring tillage (pre-till), 2) approximately 1.5-2 weeks after tillage (post-till), 3) approximately one month later (mid-season), and 4) after the last squash harvest (harvest). Sampling was done by collecting eight composite samples to a 15 cm depth, homogenizing, and dividing the sample into two subsamples. One was dried at 35ºC for at least 48 hours before grinding and sieving to 2 mm, and setting aside for inorganic N extractions, POXC analysis, and C/N analysis (Elementar VarioMAX CN analyzer, Elementar Americas). The other subsample was sieved to 2 mm and kept field-moist at 4ºC for MB and PMN determinations.
Permanganate oxidizable C
Permanganate oxidizable carbon was measured according to Weil et al. (2003). In short, 2.5 g of dry soil were reacted with KMnO4, a strong oxidizing agent. Diluted supernatants were transferred to 96-well plates and measured on a spectrophotometer at 540 nm. Absorbance was fitted to a standard curve, and calculated to determine C oxidation by KMnO4 reaction.
Soil inorganic N and PMN
Inorganic N was extracted from soil sampled using 1 M KCl and filtered through #42 Whatman papers (Robertson et al., 1999). Extractions were frozen in scintillation vials until N analysis on a Shimadzu TOC and TN analyzer (Kyoto, Japan).
In 2016, post-till soils were analyzed for PMN using a 7d anaerobic incubation (Drinkwater et al., 1996) and a 28d anaerobic incubation method (adapted from Scott et al., 1998; Prescott et al., 2005) in order to assess cover crop contributions to N cycling as well as to compare methods for this use. In the 7d anaerobic incubation, soils were submerged in 10 mL water within a test tube, purged with N2 gas, and sealed. Tubes were kept in a 37ºC incubator for exactly seven days, then extracted with 1.3 M KCl. The 28d aerobic incubations were similar; 10 mL water was added to dry soil samples until water holding capacity was reached. Tubes were loosely capped and incubated in a bin at 37ºC for 28 days. Tubes were weighed three times per week to ensure consistent soil moisture. Extractions were the same as in the 7d anaerobic incubation, and both sets of extracts were analyzed on a Shimadzu TOC and TN analyzer (Kyoto, Japan). Final values were calculated by subtracting soil inorganic N from incubation values.
Before crop planting, beds were prepared using a tractor mounted bed shaper and plastic layer (1721-D bed shaper + 1723 plastic mulch layer, Buckeye Tractor Co, Columbus Grove, OH) immediately before crop planting. Certified organic yellow crookneck squash were direct seeded and rows were covered with 1.5 m wide row cover until row cover restricted crop growth, to deter both herbivores and insect pests. When row covers were removed, plants were counted and used to adjust yield calculations.
Data were analyzed using SAS (Cary, NC). Data for 2015 and 2016 were analyzed separately due to spatial and temporal differences between years. The PROC MIXED procedure, including Tukey’s HSD, was used to detect differences in POXC, inorganic soil N, PMN, cover crop biomass, percent N, C/N, and squash yield between treatments (p < 0.10). Cover crop treatments and sampling points were fixed effects, while block was a random effect. A t-test was used compare early and late cover crop samples, as well and in-row versus between-row samples. Repeated measures for soil proxies were analyzed.
Cover crop biomass and N content
There was no difference between treatments or species at the 2015 early sampling in biomass N, percent N, or C/N. By the late biomass sampling, between row percent N across treatments and species had diminished. The C/N of the RV treatment was greater in the late sampling than in the early sampling, yet despite this and the lowered percent N over time, leaving biomass between rows to mature allowed for an accumulation in total biomass N.
Species and treatment differences were more apparent in 2016. All species in the OP treatment had higher N concentrations than those in RV or C, and a conversely lower C/N than all but hairy vetch in rows at the early termination. RV had more biomass N than OP in rows, and RV and C had more biomass N than OP. The C treatment was the only one left growing between rows at the early termination (though it was mowed), as it tolerates occasional defoliation. Between rows at the early sampling, red clover had a lower C/N and biomass N than weedsC, but these differences were not observed at the late sampling. Instead, the late C/N of red clover was lower than at the early sampling, and weed biomass N decreased, likely due to mowing and subsequent red clover growth. Thus, by the late sampling, red clover had outcompeted weeds such that it comprised 95% of the biomass N.
Permanganate oxidizable C
Few significant observations were found for POXC. In 2015, pre-till OP POXC was greater in rows than between rows, although this difference likely does not stem from treatments since OP had not yet been incorporated in rows. By harvest, RV had greater in row POXC than OP. In 2016, pre-till and post-till samplings had more POXC than at harvest across all treatments.
Soil inorganic N and PMN
In 2015, the post-till soils had greater inorganic N in rows than between rows, though this was only significant for RV. By mid-season sampling, OP between-row inorganic N was greater than in rows, and between row inorganic N was greater in RV than in OP.
In 2016, OP had more soil inorganic N than RV and C at the pre-till soil sampling. By mid-season, in row inorganic N was higher in OP than C, but between-row soils showed no differences by cover crop treatment. At harvest, soil inorganic N between rows was greater for C than in OP and RV, and was also greater than in row of the C treatment.
Potentially mineralizable N was measured only at post-till in 2016. In row trends between the two methods were similar to one another; in the 7-day anaerobic incubation, PMN was higher in C than in OP, and in the 28-day aerobic incubation C was greater than both OP and RV. Between rows, only the 7-day anaerobic incubation showed any differences; RV and C had greater PMN than OP.
We detected no effect of cover crop treatment on squash yields, likely due to large variance in the data. Squash yields were adjusted for the number of plants present in crop rows, as herbivory in 2015 and insect pests in 2016 were problematic throughout the growing seasons. However, squash yields shown were not adjusted for row centers because this space allocation was either necessary for allowing between-row space for cover crops, or unnecessary when no cover crops were left between rows (C in 2016).
Educational & Outreach Activities
Authors are in the process of writing a manuscript of this research and submitting it to peer-reviewed, soil-focused publication outlets. Over the course of the study, students in the Student Organic Farm: Planning, Growing, and Marketing class (HORT 3131) at the University of Minnesota, Twin Cities, were able to see and learn about cover cropping management systems by using these field plots as hands-on learning tools over two semesters. In addition, a SARE Professional Development Program grant allowed us the opportunity to display our work and research to immigrant and minority farmers during a two-day soil health and cover cropping workshop that occurred at the Good Acre. The field site was also a part of the UMN Student Organic Farm’s (Cornercopia) open house events.
Delaying between-row cover crop termination more than doubled the amount of biomass N provided by the RV treatment in 2015, but did not change the amount of biomass N from the OP treatment. This was achieved by biomass growth, not via decreased C/N. Results from 2016 were slightly more complex given that OP treatments were conventionally tilled (full-width), RV was mowed full-width at the early termination point due to legume maturity, and C was mowed full-width but continued growing between rows. This allowed for multiple C clippings and thus N applications, and by the late termination overall weed biomass and clover C/N had both decreased.
Soil quality as measured by POXC was steady across cover crops in both years. This suggests that none of the practices used decreased soil SOM accumulation and storage. Measures of N cycling instead seemed to provide the more sensitive explanations for the impact of cover crops on soil fertility and crop yield. Inorganic N seemed to respond to cover crop inputs when C/N were appropriately low. Alternatively, when between-row cover crops were terminated via flail mowing, between-row inorganic N increased relative to in-row. Similar trends existed in 2016 but were delayed according to higher C/N values of cover crops. It seemed that PMN was able to detect organic N additions that had not yet been mineralized, providing a helpful glimpse at the turnover time of cover crop N.
Medium red clover, though only used as a treatment in 2016, has high potential for use as a continuous between-row cover crop, or living mulch. Clover had less excess soil inorganic N at many time points over the growing season, and provided a high squash yield. Moreover, continuously living roots between row may serve to cycle N efficiently while contributing to more stable SOM pools. The ability to periodically mow red clover was useful in lowering weed presence and reducing overall crop competition.
We did not create or implement an economic analysis in this study, as it was outside of the scope of the project’s objectives. However, it is important to acknowledge that the use of legumes for N fertility provides organic growers with substantially lower fertility costs than are otherwise available. Moreover, appropriate cover crop residue management, as investigated within this study, is useful for optimizing the benefits provided by cover crops.
We believe that increased knowledge of the below-ground processes that make zone tillage beneficial for growers will aid in farmer adoption of innovative cover cropping and soil management strategies. We also acknowledge that in many horticultural systems, growers often utilize living mulches, such as white clover, and zone tillage to aid in weed maintenance and crop harvests. Our work validates these cultural practices from scientific points of view.
Areas needing additional study
Further research on between-row N cycling and movement will aid in understanding the effect of labile N sources on nearby crop N provisioning. Work that also compares belowground sources of N between annual and perennial living mulches might also inform future management practices and cover crop choices.