Impact of cover crop species, termination stage and termination method on soil nitrogen, organic carbon and enzyme activities

Abstract

Soil is an essential resource and it is vital to manage its quality in order to improve agricultural yield and environmental quality. A number of methods have been used for decades to improve soil quality including cover cropping. Cover cropping can serve as a sustainable approach to combat soil degradation, provide organic carbon and improve microbial processes and fertility in agricultural soils. Thus, it is crucial to adopt the best cover crop management practices in order to obtain their full benefits. Although, there is a considerable body of literature on cover crop use and management, research assessing how specific cover crop species, termination stage and termination method affect soil organic carbon, nitrogen and enzyme activities remains limited. Currently, no combined study of such processes has been reported in Africa, including the South African cropping systems. Two separate studies were conducted in the greenhouse and field in the Western Cape Province to examine the impact of two termination stages (vegetative and flowering) on the biomass, total C, N and C:N ratios of four cover crops. The study assessed the immediate effect of living cover crops and residues, two termination stages and two termination methods (slash and spray) on soil β-glucosidase, phosphatase and urease activities as well as soil pH, soil organic carbon (SOC) and total soil N. Species tested as cover crops were, grazing vetch (Vicia dasycarpa Ten.), field pea (Pisum sativum), oats (Avena sativa L.), rye (Secale cereal L.) and a control (no cover crop). The experimental design was a randomized block design with 3 replications. Soils were sampled at the beginning of the experiment, at kill and at one year after cover crop kill. Cover crops were sampled at kill. The first phase of the study (greenhouse experiment) was carried out from 2016 to 2017, with soils obtained from the Agricultural Research Council (ARC) Nietvoorbij Research Farm. The second phase of the study (field experiment) was carried out at two sites, ARC Nietvoorbij and Bien Donne Research Farms, from 2017 to 2018. It was observed in the greenhouse study that delay in termination of cover crops till flowering stage significantly increased total C and C:N ratio content of all the cover crops. However, delaying termination from vegetative till flowering stage decreased N in the tissue of pea, oats, vetch and rye by 59%, 65%, 44% and 56%, respectively. Cover crop presence in the soil had no effect on soil pH. At kill, living cover crop species did not influence SOC and total soil N but stimulated β-glucosidase, urease and phosphatase activities compared to the control. At one year, rye, oats and pea residues increased SOC by 8%, 7% and 4%, respectively. However, only pea and vetch residues increased soil N level in the short-term. At one year, β-glucosidase activity was greater under rye (77.58 μg PNP g-1 soil h-1), oats (76.53 μg PNP g-1 soil h-1) and pea (74.64 μg PNP g-1 soil h-1) residues than vetch (67.99 μg PNP g-1 soil h-1) and control (61.06 μg PNP g-1 soil h-1). Unlike urease, all cover crop residues had a positive impact on phosphatase activity at one year. Termination of cover crops at flowering stage resulted in greater β-glucosidase activity and SOC levels than the vegetative stage at one year. Compared to flowering, termination at vegetative stage improved soil N levels and phosphatase activity at both sampling times. Termination by slash had a positive impact on SOC and β-glucosidase activity compared to spraying. However, termination method had no effect on soil N, urease and phosphatase activity at one year. The significant interaction (P<0.05) of sampling time, cover crop and termination stage effects on SOC, total soil N, β-glucosidase and phosphatase activities observed in this study indicates that these management approaches can maximize cover crop benefits and improve soil fertility in the short-term. Field experiment results indicate that delaying termination from vegetative till flowering stage increased the biomass yield, total C and C:N ratios of vetch, pea, oats and rye, respectively, while their tissue N decreased, at both sites. Cover crop biomass yields at the Nietvoorbij site varied from 207 - 4480 kg ha-1 while those at the Bien Donne site varied from 835 – 9891 kg ha-1 with indices being higher under rye and oats than pea and vetch. Cover crop presence reduced soil pH at both sites. At kill, living rye and pea considerably increased SOC at the Bien Donne site, while at one year, the residues of oats raised SOC by 7% and 9% at Nietvoorbij and Bien Donne sites, respectively. Generally, cover crops were slow to impact SOC and the activities of β-glucosidase, urease and phosphatase, at both sites. Total soil N varied and was elevated by all the tested living cover crops and residues, at both sites. Results at both sampling times showed that cover crop termination at the flowering stage promoted the activities of the three enzymes than the vegetative stage, at both sites. At kill, higher SOC and total soil N was also observed at flowering stage compared to vegetative stage. However, at one year, total soil N was greater at vegetative stage than the flowering stage at both sites. Although, termination methods had no effect on SOC and total soil N at the Nietvoorbij site, termination by slashing favoured SOC compared to spraying while total soil N was more promoted by spraying than slashing at the Bien done site. Soil enzymes did not respond to termination methods at the Bien Donne site, whereas slash stimulated β-glucosidase, urease and phosphatase activities than spraying at the Nietvoorbij site. The significant interaction (P<0.05) of sampling time, cover crop, termination stage and termination method effects on soil N level observed at both sites of this study (field and the greenhouse studies), indicates that these management factors can optimize cover crop benefits and improve soil fertility. However, results from this field study indicate that longer time spans may have been needed to see the impact of cover crops on soil enzyme activities and SOC levels in the Cape Mediterranean environment.