Cover crop enzyme activities and resultant soil ammonium concentrations under different tillage systems

Highlights

• Cover crop residue enzyme activities were impacted by cover crop species type.

• No-tillage increased carbon cycling enzyme activities on decomposing cover crop.

• Soil ammonium concentrations were related to cover crop residue enzyme activities.

• Cereal rye could potentially immobilize nitrogen during corn nitrogen demand.

Abstract

Increased interest in managing agroecosystems for soil health has led to adoption of conservation management practices such as cover cropping and no-tillage. There is a dearth of knowledge surrounding cover crop residue N cycling and release, impacting subsequent N availability for the cash crop after cover crop termination. The microbial degradation of plant material and subsequent nutrient release is mediated by extracellular enzymes. This study used litter bag methodology to investigate the enzyme activities of three cover crop residues (cereal rye (CR) (Secale cereale L.), hairy vetch (HV) (Vicia villosa Roth), and a CR/HV mixture) in two tillage systems (no-tillage and reduced tillage) during the cover crop decomposition period in a corn (Zea mays L.) agroecosystem. The dynamics and amounts of β-glucosidase (EC 3.2.1.21) and urease (EC 3.5.1.5) enzyme activities on cover crop residue were quantified to determine if enzyme activities or stoichiometry are dependent on litter quality or can indicate N cycling potential. Results revealed that cover crop and tillage significantly impacted urease and β-glucosidase residue enzyme activity dynamics and amounts (p < 0.05). β-glucosidase activity of CR, HV, and a CR/HV mixture decreased by 76 %, 81 % and 77 % during the 2016 decomposition period, respectively. On average, urease activities of cover crop residues and soil ammonium concentrations increased during this period. In 2017, urease activities significantly decreased in all treatments (p < 0.05). β-glucosidase enzyme activity significantly decreased from 38 days after termination to 123 days after termination in all cover crop and tillage treatments (p < 0.05). Urease activities were significantly higher in the tillage treatments compared to no-tillage at the beginning of the 2017 decomposition period (p < 0.05). Soil ammonium concentrations significantly increased by 38 % across all treatments in 2017 during the decomposition period and concentrations were 12 times higher in 2017 at corn maturity compared to 2016. These results indicate potential for cover crops to stimulate residue β-glucosidase activity, suppress urease activity and ammonification, and increase soil N immobilization during corn N demanding growth stages. Furthermore, this study indicates that growers should consider optimum cover crop winter biomass accumulation levels to avoid potential N immobilization.

Introduction

There is a growing interest in improving soil health of agroecosystems through the inclusion of cover crops and reduced and no-tillage residue management strategies. Several ecosystem services have been associated with cereal rye (Secale cereale L.) (CR) as a cover crop, such as its potential to increase soil organic carbon (C) (Beehler et al., 2017; Jarecki and Lal, 2003), reduce soil erosion and compaction (Calonego et al., 2017; Villamil et al., 2006), and enhance water infiltration (Folorunso et al., 1992; McVay et al., 1989). Of late, the ability of CR to scavenge inorganic nitrogen (N) from the soil solution that would otherwise be susceptible to leaching has become relevant due to the relationship between agricultural drainage and hypoxia in major freshwater bodies (Kaspar and Singer, 2011). Though this scavenging of N benefits water quality by decreasing potential nitrate release to water systems (Ruffatti et al., 2019), there are barriers to widespread cover crop adoption (Roth et al., 2018). The literature has demonstrated a potential for reduction in yield for corn (Zea Mays L.) following CR termination (Kaspar and Bakker, 2015; Kramberger et al., 2014, 2009), sometimes as a result of CR allelopathic effects (Weston, 1996). The addition of C from cover crops after termination during the corn cash crop growing season could increase microbial N demand and result in soil N immobilization during corn N demand. One of the major barriers to cover crop adoption is the lack of understanding of the fate of cover crop N after termination (Snapp and Borden, 2005; Snapp and Fortuna, 2003). Furthermore, reduced and no-tillage applications that are routinely implemented with cover cropping may further increase N immobilization due to residue accumulation. The timing of N release to the soil solution from cover crop biomass and the cycling dynamics of that N after cover crop termination are relatively unknown. There is also a dearth of knowledge on the synchrony of cover crop biomass N release to the soil solution and the N demand of a growing corn crop. Improving our understanding of microbial community responses during cover crop decomposition will increase knowledge of cover crop N fate after termination. This could lead to heightened adoption of cover crops and increased soil health.

Understanding the activity of the microbiota on decomposing cover crop residue will provide critical insight into the rate of cover crop biomass N release and cycling during the growing season of the subsequently planted cash crop. Extracellular enzyme activities in environmental samples, especially soils, have been quantified to understand the biochemistry of decomposition and nutrient cycling (Skujins, 1978). Residue (litter) enzyme activities have been studied in decomposing vegetation in forest (Purahong et al., 2015), restored forest (Smith et al., 2015), and pasture (Dilly et al., 2003, 2007) ecosystems, as the microbial degradation of organic material is mediated by extracellular enzyme production (Sinsabaugh et al., 1991). However, the dynamics, amounts, and stoichiometries of plant residue enzyme activities have seldom been studied in agroecosystems (Dilly et al., 2007), despite their effectiveness for evaluating C and N processing at the microbial level during residue decomposition (Sinsabaugh et al., 1991). As extracellular enzyme production mediates nutrient acquisition from organic material (Tabatabai et al., 2010), the activities of extracellular enzymes are indicators of microbial nutrient demand (Caldwell, 2005; Moorhead and Sinsabaugh, 2006).

The activities of β-glucosidase (EC 3.2.1.21) and urease (EC 3.5.1.5) are estimators for the degradation of C polymers and the release of ammonia, respectively (Das and Varma, 2010; Dilly et al., 2003). β-glucosidase is found among plants, soils, fungi and bacteria (Veena et al., 2011) and is crucial in the catalysis of biodegradation of β-glucosides present in plant material (Martinez and Tabatabai, 1997). β-glucosidase has a role in the last step of C polymer degradation and hydrolyzes cellobiose residue resulting in glucose as a final product (Adetunji et al., 2017), which is an important energy source for microbial growth and activity. Urease is widely distributed in nature (Follmer, 2008) and catalyzes the hydrolysis of hydroxyurea, resulting in the production of carbon dioxide and ammonia (Adetunji et al., 2017; Pettit et al., 1976). This resulting ammonia can be lost to the atmosphere through volatilization or rapidly converted to ammonium through ammonification.

Despite β-glucosidase and urease residue enzyme activities being responsive to easily decomposable organic substrates and soil nutrient conditions (Dilly and Nannipieri, 2001), the dynamics of these enzymes on cover crop residue following cover crop termination have not been quantified over time. Understanding the dynamics of enzyme activity on cover crop residue could give insight into the timing of cover crop residue C influx into the soil matrix (Tabatabai et al., 2010), which has potential to lead to N immobilization and decreased plant available N supply for the cash crop. Furthermore, the assessment of cover crop residue enzyme activities in conjunction with measurements of cover crop biomass accumulation and soil ammonium concentrations over time could elucidate the timing of cover crop biomass N release and how it relates to the N demand of corn during growth and development.

The primary objective of this study was to evaluate the impact of different cover crop species (CR, hairy vetch (HV) (Vicia villosa Roth), and a CR/HV mixture) and tillage regimes (15 cm spring reduced tillage and no-tillage) on enzyme activities of decomposing residue and quantify soil ammonium concentrations after cover crop termination during two corn growing seasons. We hypothesized that residue β-glucosidase and urease activities would be influenced by cover crop species, tillage application, and stage of cover crop decomposition as related to C input because of differing cover crop C:N ratios and no-tillage. Cover crops differ in quality traits (C:N ratio) based on species, with CR commonly having a wider C:N ratio than legumes such as HV. This study included a mixture treatment consisting of CR and HV to assess whether the addition of HV increases, decreases, or has no impact on enzyme activities of CR litter. Regarding tillage system, no-tillage has potential to lead to C accumulation in the surface soil. This is unlike spring tillage which mixes soil and decomposing residue (Mbuthia et al., 2015; Sievers and Cook, 2018). We also hypothesized that soil ammonium concentrations would be related to levels of residue urease activities as this could be the rate limiting step before ammonium production.