A field trial in Ely, Cambridgeshire was set up to investigate the soil moisture and nitrogen dynamics of a frost sensitive cover crop compared to a control of an over winter stubble. Cover crops were established in late summer following wheat harvest and a summer tillage operation. Soil and aboveground biomass sampling commenced in September 2017 was continued at 2–3 week intervals until May 2018. The results highlight the fast growth of cover crops which rapidly reduced the total oxides of nitrogen present in the soil by late autumn. During winter a gradual increase in total oxides of nitrogen is measured as the cover crop residue is mineralized. The use of the frost sensitive cover crop permitted ease of management and termination, though it is not completely reliable and termination times can vary from year to year

Hannam, Jacqueline A.

Simmons, Robert W.

Storr, Tom

English

Cranfield CERES

Soil Water and Available Nitrogen during Cover Crop GrowthBy T STORR, R W SIMMONS and J HANNAMCranfield University, College Rd, Cranfield MK43 0AL, UKCorresponding Author Email: Tom.Storr@cranfield.ac.ukSummaryA field trial in Ely, Cambridgeshire was set up to investigate the soil moisture andnitrogen dynamics of a frost sensitive cover crop compared to a control of an overwinter stubble. Cover crops were established in late summer following wheat harvestand a summer tillage operation. Soil and aboveground biomass sampling commencedin September 2017 was continued at 2–3 week intervals until May 2018. The resultshighlight the fast growth of cover crops which rapidly reduced the total oxides ofnitrogen present in the soil by late autumn. During winter a gradual increase in totaloxides of nitrogen is measured as the cover crop residue is mineralized. The use of thefrost sensitive cover crop permitted ease of management and termination, though it isnot completely reliable and termination times can vary from year to year.Key words: Cover crops, Soil moisture, NitrogenIntroductionThe use of over winter cover crops is gaining interest in the UK and has been supported bythe Basic Payment Scheme (BPS) as an ecological focus area (EFA). The most recent availabledata shows that the total area of catch crops and green cover increased by 45% under the BasicPayment Scheme between 2014/15–2015/16 (DEFRA, 2017). Cover crops can be utilised toimprove soil structure (Stobart et al., 2015), soil erosion control (Panagos et al., 2015), aidweed suppression (Brust et al., 2014) and soil moisture management (Dabney et al., 2001).Cover crops also perform an important ecosystem service by reducing nitrate leaching by 20–90% when compared to a bare soil (Justes et al., 2012) .Following cover crop growth and decomposition, the nitrogen contained with the cover cropbiomass can be mineralized for the following cash crop thus having a green manure effect(Thorup-kristensen et al., 2003). The nitrogen dynamics of the cover crop – cash crop systemare affected by numerous variables; C:N - linked to cover crop species (Couëdel et al., 2018)and maturity, termination date, use of tillage (Wyland et al., 1996), weather and soil type.Cover crop growth can also reduce soil moisture and may be used to dry out heavy land (Shahet al., 2015).The research conducted compared the concentration of available of nitrogen and the soilmoisture in a high organic matter soil between cover crops and an over wintered stubblecontrol. This will allow a greater understanding of the nitrogen and water dynamics of a systemusing frost sensitive cover crops that will self-senesce over winter.Materials and MethodsThe trial field is located at Littleport, near Ely on an organo-mineral top soil of theAdventurers’ series which is approximately 28% w/w soil organic matter. Following wheatharvest in late summer 2017 the trial area was cultivated using a progressive cultivator(Keeble). A triplicate of cover crop plots (24m × 100m) were established alternating withequally sized control plots. The cover crop mixture of 60% black oats (Cadence), 35% oilradish (Final) and 5% white mustard (Braco) were established on 24 August 2017 and rolled.The control plots were cultivated and were initially bare ground but subsequently grew withwheat volunteers. The field trial was terminated on 21 April 2018 with glyphosate at 4L/Ha.Forage maize (P8200) was then zero tilled into the desiccated cover crop on 20 May 2018.Three soil samples at 0–15cm depth per plot were randomly collected and mixed before thedetermination of total oxides of nitrogen (TON) and gravimetric soil water content. TON wasdetermined following a standard method and extraction with 2mol KCl. Above ground biomasswas cut from a 0.25m2 quadrat, dried and weighed. Soil and above ground biomass sampleswere taken at 2–3 week intervals between September 2017 and May 2018. For brevity rainfalldata is not shown but was recorded by an on farm weather station to evaluate any potentialinfluence on soil moisture and nitrogen. Results between the treatments were analysed usingthe students T-test.ResultsFigure 1 highlights the above ground biomass of the frost sensitive cover crop and the control(naturally regenerated wheat volunteers). Peak cover crop biomass is achieved in late autumnand then recedes as some of the frost sensitive plants begin to senesce.Figure 1: Above ground dry biomass sampled between September 2017 and May 2018. Significantdifference (t-test; P<0.05) between treatments on the same date is denoted by an *. n = 3 per treatmentat each sample point. Error bars = standard error.Note: above ground biomass samples were not collected on the 4 September 2017 as the cover crop hadonly just emerged.A rapid depletion of TON in the soil is observed in the cover crop treatment (Figure 2) inautumn before a gradual increase in TON availability from mid-winter onwards. A largeincrease in TON is measured in the soil following termination for both the control and covercrop treatment.During the growth of the cover crop and into late-winter Figure 3 shows that cover cropsreduce, although not significantly the gravimetric soil moisture content. At the last samplingpoint the difference between the cover crop treatment and the control is greatest, but again notsignificantly different possibly due to the large variability in the control treatment.Figure 3: Gravimetric soil moisture sampled at 0-15cm between September 2017 and May2018. Significant difference (t-test; P<0.05) between treatments on the same date is denotedby an *. n = 3 per treatment at each sample point. Error bars = standard error.Figure 2 Total oxides of nitrogen as sampled at 0–15cm between September 2017 and May 2018.Significant difference (t-test; P<0.05) between treatments on the same date is denoted by an *. n = 3per treatment at each sample point. Error bars = standard error.DiscussionFigure 1 highlights the rapid growth of the cover crops over the autumn months which isexpected given that fast growing cover crop species (black oats, oilseed radish and whitemustard) were selected for the trial. The slow but increasing biomass of the control indicatesthe regrowth of wheat volunteers into the control plots. As expected the biomass of the covercrop treatment decreases after mid-December as the species selected were all frost sensitive.This frost sensitive trait of the cover crops is desirable from a management point of view as asmaller cover crop canopy at termination would prevent the shading of weeds and volunteersfrom herbicide. During January, Figure 1 shows that the cover crop aboveground biomassbegins to plateau around 2 t ha-1 which is explained by the mustard species becoming lignifiedbut also it was observed that a small but noticeable number of oilseed radish plants were notterminated by the frost. This is unexpected as the oilseed radish variety selected had been usedin the previous season where it was effectively controlled by the frost. As the cover cropbiomass gradually decreases for the remainder of the sampling period, the wheat volunteers inboth the cover crop treatment and control continue to grow.TON present at the start of the field trial is similar in both treatments. Though as the autumnprogresses, Figure 2 highlights the large differences in TON present in the soil between thecover crop treatment and the control. The cover crop treatment has rapidly reduced TONpresent in the soil compared to the control treatment, which is an over winter stubble withwheat volunteers. As the wheat volunteers emerge in the control a steady reduction in TON ismeasured and achieves a similar minimum TON as the cover crop treatments. Research byBaggs et al. (2000) also reports that naturally regenerated vegetation (volunteers) are aseffective as cover crops at retaining N over winter . However the reduction of soil TON in thecover crop treatment is a achieved at a much greater rate than the control containing volunteers– the rate of nitrate reduction was not reported in the study undertaken by Baggs et al.(2000).The differences in rate of TON depletion of the soil are inversely related to theaboveground biomass (Figure 1). As the cover crops senesce and their aboveground biomassdecomposes a gradual increase in TON is observed for the remainder of winter and early spring,with the exception been measured on the 12 April 2018. The reduction in TON measured onthis date in both the cover crop treatment and the control, may be explained by high soilmoisture recorded on the same date (Figure 3). Weier (1993) reports that denitrification canresult from high soil moisture contents as oxygen is reduced to the soil microorganisms.Secondly, the rainfall prior to sampling on the 12 April may have led to the leaching of mobileavailable nitrogen through the soil profile. Additionally in the control plots there is an increasein above ground biomass to the 12 April 2018. After 19 days following termination withherbicide there is rapid increase in TON from both the cover crop treatment and the controlplot when measured on the final sampling date 10 May 2018. Presumably this is due to thenitrogen mineralised from the plant biomass in both the cover crop treatment and control.Following both the control and cover crop treatment, similar levels of TON are returned tothe soil in May when compared to the start of the field trial in September 2017. The return ofthis nitrogen may be manipulated depending on cover crop species selection. Cover cropspecies may be differentiated by their ability to tolerate cold temperatures and may begin tosenesce in late winter/ early spring essentially terminating the cover crop and starting theprocess of decomposition. The timing and effectiveness of using cold temperatures to terminatea cover crop is variable and not always reliable. Vincent-Caboud et al. (2017) highlights thatcover crop regrowth and/ or incomplete termination can be limitation of relying solely on frostkilled cover crops in an organic farming system, especially under no-till circumstances. Thebenefit of using a frost sensitive cover crop may i) allow reduced application of herbicide(Kabir & Koide, 2002) ii) prevent trafficking fields earlier in the year when they are likely tobe at field capacity for cover crop termination purposes iii) allow some nitrogen to becomeavailable earlier in the season for initial cash crop growth. With the cover crop dying backearlier in the growing season, there is a risk that nitrate leaching would still be a problem. Thisrisk may be reduced by allowing wheat volunteers to continue growing in the cover crop (aspermitted in this trial) or use of a winter hardy cover crop species.From late autumn until the end of winter soil moisture in the cover crop treatment wasreduced, although not significantly at most sampling time points, when compared to the controlplot. During autumn average temperatures remained between 10–15°C and given the exposedand windy location of the trial it can be expect these factors helped drive transpiration from thecover crops, thus increasing water uptake from the soil. Furthermore, the use of cover cropfollowing cultivations can improve infiltration rate when compared to no cover crops (Nuneset al., 2018) thus moving water through the soil profile and below the 15 cm depth sampled.Soil moisture in the cover crop treatment declines rapidly at the end of April and early May,this may be attributed to the greater evaporation from the exposed soil surface as only lignifiedmustard stems and some wheat volunteers remain, compared to the larger wheat volunteerpopulation in the control.In conclusion, the use of frost sensitive cover crops facilitated the earlier return of TON to thesoils, following the majority of the cover crop biomass senescing in the cold temperatures. Thefast growth rate of the cover crops quickly and significantly reduced the TON present in thesoil in autumn and this was maintained until early January. By reducing TON present in thesoil over late autumn and early winter the risk of leaching is reduced for part of the winterperiod. However, from January onwards there is a trade-off; the nitrogen captured by the covercrops begins to return to soils thus benefitting early crop growth and there may be reducedreliance on chemical control of the cover crop. But it must also be acknowledged that nitrogenis returning to the soils at a time when there is still a risk of leaching in late winter and earlyspring before the next crop is planted.AcknowledgementsThanks are extended to my sponsors G’s Growers Ltd. In particular Charles Shropshire andAndy Knaggs who have supported the research ideas proposed and organised the fieldoperations for the trial. Agrii provided seed for the trial.ReferencesBaggs E M, Watson C A, Rees R M. 2000. 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Soil water and available nitrogen during cover crop growth

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Soil water and available nitrogen during cover crop growth

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