Tillage management exerts stronger controls on soil microbial community structure and organic matter molecular composition than N fertilization
Introduction
Agricultural management such as nitrogen (N) fertilization and tillage practices have been applied to increase agronomic productivity, but they also markedly altered soil biophysical and geochemical environment (Shakoor et al., 2021, Wuaden et al., 2020). Conventional tillage management (i.e., moldboard plowing) has been found to result in soil erosion, disrupted aggregate structure and severe nutrient loss (Karlen et al., 2013). Instead, conservation tillage (i.e., reduced, minimum or no tillage) typically requires that at least 30% of the soil surface is covered with crop residues after planting. This type of management has effectively reduced physical disturbance to soil (Liu et al., 2014, Madejón et al., 2009) and elevated soil carbon storage (Shrestha et al., 2015). However, other studies found that conservation tillage may reduce soil carbon as the retention of crop residues on soil surfaces decreases the incorporation of carbon inputs to soil, or reduces carbon inputs if crop residue production is lower compared to conventional tillage systems (Angers et al., 1997, VandenBygaart et al., 2002). N fertilization has been reported to enhance microbial activity and biodegradation of soil carbon (Chen et al., 2021, Khan et al., 2007, Manna et al., 2006). Yet, some studies observed that N fertilization increased crop residue production up to the optimal agronomic N rate (Majumder et al., 2007, Poffenbarger et al., 2017), resulting in soil carbon accrual (Batlle-Bayer et al., 2010, Morell et al., 2011). An investigation on a barley-based system combining N fertilization and tillage practices has found that the increase in carbon inputs with N fertilization was more prominent under no tillage than conventional tillage (Morell et al., 2011). This implies more pronounced soil carbon sequestration under the combination of N fertilization and conservation tillage (Álvaro-Fuentes et al., 2012). Overall, N fertilization and tillage management may potentially alter soil carbon storage antagonistically. However, mechanistically, more detail on how the interaction of various tillage and N fertilization constrains soil carbon storage and cycling is needed.
The composition and degradation of soil organic matter (OM) shift markedly under various agricultural management practices such as different tillage and N additions (Dieckow et al., 2006, Pisani et al., 2016, Shrestha et al., 2015). Compared to conventional tillage, no tillage increased the proportion of cellulose-type components (O-alkyl carbon), while it reduced the content of lipid components (alkyl carbon), and overall exhibited a lower decomposition stage of soil OM in the top (0–10 cm) soil layer (Shrestha et al., 2015). This is likely due to altered microbial decomposition and processing caused by shifts in microbial community composition and diversity as a higher ratio of fungi over bacteria (Panettieri et al., 2020) and higher bacterial diversity (Quadros et al., 2012) were observed under reduced/no tillage relative to conventional tillage. Conservation tillage (i.e., no tillage) was also found to elevate microbial abundance (Wang et al., 2014) likely associated with the altered substrate availability and physiobiological environment (Ghimire et al., 2014, Lupwayi et al., 2017, Rahman et al., 2008). An investigation of increasing N fertilization rates under conventional tillage reported enhanced soil OM decomposition and lower leaf- and root-derived lipid contents relative to non-fertilized plots (Man et al., 2021a), likely due to altered plant litter/substrate chemistry (Cai et al., 2015, Conti et al., 1997) and/or changes in microbial processing (Jian et al., 2016). This may be also related to the changes in microbial community composition under N addition such as higher fungi over bacteria ratios (Man et al., 2021a, Zhao et al., 2014) and an increase in microbial diversity (Ding et al., 2017, Yao et al., 2021). It is also important to note that N addition may exert varying controls on soil microbial community structure in the long-term in cropping ecosystems (Geisseler and Scow, 2014, Tosi et al., 2021). However, another study conducting N fertilization treatments based on no tillage systems did not observe any significant changes in soil OM compounds between non-fertilized and fertilized soils (Dieckow et al., 2006). Piazza et al. (2019) investigated microbial dynamics and also found that the shifts in fungal community structure induced by N fertilization differed under conservation vs. conventional tillage. These findings collectively suggest that the manner by which N fertilization alters soil OM biogeochemical processes may be dependent on the applied tillage practices. These observations are likely because the controls of N fertilization and tillage practices on soil OM cycling may be antagonistic and interact or offset each other. As such, it’s hypothesized that the combination of these two treatments can result in interactive controls on soil OM dynamics (Morell et al., 2011). However, a deeper mechanistic understanding is required especially on how the combination of N fertilization and tillage management alters soil OM biogeochemistry trajectories, which may provide specific perspectives on soil carbon stabilization/destabilization under multi-practice agroecosystems.
To improve the understanding of the interacting controls of N fertilization and tillage systems on soil carbon dynamics, soils collected from a 24-year experiment involving contrasting N fertilization rates (low and moderate N) and tillage management (conventional and conservation) were used to investigate OM composition and microbial community changes. Soil samples were analyzed for organic carbon and N content, and OM composition using targeted chemical analyses and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Quantitative PCR (16S and 18S rRNA) and DNA sequencing (16S rRNA and ITS) were applied to evaluate changes in microbial abundance and community composition, respectively. We hypothesize that 1) N fertilization and tillage systems change soil OM composition, likely due to altered microbial communities and the biodegradation of specific OM components under these treatments; 2) With the observed interaction between N addition and tillage practices (Man et al., 2021a, Dieckow et al., 2006), it is expected that N fertilization-driven changes in soil OM components and degradation, as well as microbial communities, differ under conservation vs. conventional tillage.
Section snippets
Site description and sample collection
The experiment comprises of a subset from a long-term trial established in 1995 at the University of Guelph Ridgetown Campus, Ridgetown, ON, Canada (42°26´N, 81°53´W). The soil type is an Orthic Humic Gleysol clay loam according to the Canadian System of Soil Classification with neutral pH (6.45–6.85) in the top 0–15 cm layer (Van Eerd et al., 2014). The climate is warm humid continental climate with mean annual temperature of 9 °C and mean annual precipitation of 895 mm (from year 1981 to
Soil organic carbon, nitrogen (N) content and organic matter (OM) composition
Soil organic carbon concentrations did not significantly differ across treatments although they were slightly lower under conventional tillage with low N rate compared with other treatments (Fig. 1a). We only observed higher soil N content under conservation compared with conventional tillage with low N level (P < 0.05; Tables S1 and S2). Soil organic carbon to N ratio (C:N) was not significantly altered across treatments (Tables S1 and S2). The soil OM compositional change was mostly observed
Soil organic carbon and organic matter (OM) compositional change under different tillage and N fertilization treatments
Soil carbon concentrations did not significantly vary under long-term tillage practices and N fertilization rates (Fig. 1a). These results are consistent with other tillage studies, a 19-year investigation in central Italy clayey soils (Barbera et al., 2012) and a 25-year study in Southern Ontario (Deen and Kataki, 2003) that also reported similar soil carbon concentrations in the upper mineral soil (0–10 cm). Moreover, soil carbon concentrations did not significantly change in upper soil
Conclusions
Despite similar soil organic carbon concentration, soil OM composition was significantly altered by tillage management and N fertilization rates. Conservation tillage resulted in higher contents of specific OM components (i.e., long-chain acyclic lipids, cyclic lipids, simple sugars, cutin- and suberin-derived compounds) compared with conventional tillage for both N addition rates. In contrast, conservation tillage resulted in lower lignin-derived compounds relative to conventional tillage for
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada Climate Smart Soils CREATE program, Discovery Grants to K.E.D. and M.J.S., and the Canada First Research Excellence Fund Food from Thought program. We sincerely thank Dr. Ronald Soong for assistance with NMR acquisition. M.J.S. thanks NSERC for support via a Tier 1 Canada Research Chair in Integrative Molecular Biogeochemistry. M.M. thanks the University of Toronto for the Connaught International
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