May conservation tillage enhance soil C and N accumulation without decreasing yield in intensive irrigated croplands? Results from an eight-year maize monoculture
Introduction
Agriculture has to play a central role for mitigating the unfavorable effects of climate change, by reducing Greenhouse Gases (GHGs) emission to the atmosphere and increasing carbon (C) storage in agricultural soils (UNEP, 2017). At the same time, major needs are restoring soil health conditions and biodiversity (Bouma and McBratney, 2013), as well as enhancing land productivity potential and food production in the context of a rapidly growing human population (Godfray and Garnett, 2014). In the last decades, conventional and intensively managed agro-ecosystems, which depleted most of soil organic matter (SOM), have been widely indicated as major responsible for soil fertility degradation (Triplett and Dick, 2008). Around 45 % of soils in Europe currently have low to very low SOM content (Jones et al., 2012). This is particularly the case of intensively tilled and highly fertilized soils in temperate areas, where predicted future losses may reach the 24 % of the total current SOM stock (Wiesmeier et al., 2016). Thus, introducing sustainable soil management systems able to increase SOM stocks is crucial for enhancing soil fertility and ensuring productivity potential and incomes for farmers (FAO, 2011).
Soil organic matter is mainly composed by soil organic C (SOC), which is a major actor in the natural carbon cycle and represents the largest storage of terrestrial C. Soil organic C also affects soil functions and processes that regulate nutrients release for plants (Troeh and Thompson, 2005). Therefore, increasing the potential of agricultural soils to sequester CO2 as SOC is a key way not only to mitigate global GHGs emission and climate change, but also to enhance soil fertility and food production (Lal, 2004). Similarly, enhancing soil total N (STN) allows to increase productivity potential of soil, as well as to reduce dependency of farmers on chemical fertilizers, which has been recognized as major sources of N pollution into water and air (Hansen et al., 2017).
In the recent past, many studies demonstrated that the increase in SOC and STN levels is primarily related to the return of fresh organic matter to the soil (Kong et al., 2005; Ogle et al;, 2012). Agricultural practices that increase inputs of fresh organic matter to the soil are likely to positively affect SOC and STN levels in agricultural soil (Lützow et al., 2006). The addition of organic fertilizers may further support this process (Bhattacharya et al., 2016). Conversely, physical disturbance of the soil due to tillage usually increases microbial activity and SOM decomposition, thus reducing SOC and STN accumulation (Ogle et al., 2012; Perego et al., 2019). Reducing soil disturbance, which delays the rate of soil aggregates break-down, may stabilize SOM and limit SOC and STN losses (Six et al., 2000a; Wiesmeier et al., 2019).
A recent EU report (European Commission, 2016) defined various strategies to improve C and N cycling at agricultural level. This report includes a series of sustainable (agro)ecosystems managements, for preserving existing C stocks and removing at the same time C from the atmosphere, while having a positive impact on food security, agro-industries, water quality and the environment (Tilman et al., 2002). Conservation Agriculture (CA) has been repeatedly indicated as a recommended way to pursue those objectives (FAO, 2013). However, CA practices in Europe have not taken off yet in any significant scale, especially in intensive agricultural scenarios, and CA adoption is still an ongoing process whose long-term feasibility needs to be evaluated (Soane et al., 2012; Corsi, 2019). In addition, the beneficial effect of CA on crop yield and long-term C sequestration potential has been recently argued (Ogle et al;, 2012; Powlson et al., 2014).
The main objective of this study was to test the adoption of conservation tillage (i) under intensive agro-ecosystems based on maize (Zea mays L.) monoculture, with (ii) high N fertilization rates (> 200 kg N ha−1 yr−1) and organic matter inputs, (iii) permanent optimum soil water moisture due to irrigation, and (iv) temperate climate. Specific objectives were: (1) to measure the effect of contrasting tillage systems (i.e. no-till, minimum tillage, and conventional tillage) on grain yield and biomass input to the soil (biomass return) during an 8-year maize monoculture, and (2) to examine how those tillage systems affect the evolution of SOC and STN levels (i.e. concentration and mass). Physical fractionation techniques were also employed in order to clarify mechanisms behind SOC and STN stabilization in soil aggregates. The following hypotheses were tested: (i) introducing conservation tillage (i.e. no-till and minimum tillage) within intensive agricultural context devoted to maize monoculture - as that of the Po Valley - does not reduce grain yield and biomass return, and (ii) SOC and STN concentration and mass, as well as SOC and STN stabilization within soil aggregates, are highly enhanced under CA in this context.
Section snippets
Experiment and treatments
The field experiment was set up in a commercial farm in Luignano di Sesto ed Uniti, (45°12′24.1″ N, 9°53′23.3″ E; 52 m above sea level), Cremona, Po Valley, Northern Italy. Soil was coarse loamy mixed, superactive, mesic Aquic Haplustepts (Soil Survey Staff, 2014). Main physic-chemical properties of soil are shown in Table 1. Climatic data were collected from the meteorological station positioned in the field. Mean annual temperature ranged between 12.6 and 16.1 °C and cumulative annual
Grain yield and N uptake
Grain yield of maize was significantly affected by tillage treatment (T) and experimental year (Y), as well as by the interaction T × Y (Table 2). Thus, differences among treatments occurred in 7 out of the 8 years of the experiment and varied from year to years (Table 3). Overall, the highest grain yield occurred under MT in 2016, while the lowest under CT and NT in 2013. Specifically, in 2010 and 2011, grain yield was significantly lower under NT than under CT (−16 % and −17 %) and MT (−13 %
Response of maize yield and N uptake to minimum tillage and no-till
In the present study, tillage system significantly affected maize yield, supporting the claim that tillage of soil plays a major role to affect a number of physic-chemical parameters (Tabaglio et al., 2009) that may greatly impact on root development (Fiorini et al., 2018) and plant growth, thus driving changes in crop yield (Fuentes et al., 2009; Aziz et al., 2013). Our results showed that the average grain yield and N uptake of an 8-year maize monoculture decreased in the order MT > CT > NT.
Conclusions
After our 8-year tillage comparison on an irrigated maize monoculture, we concluded that minimum tillage (MT) is a valuable option to increase maize yield and biomass return compared with conventional tillage (CT). This is not the case for no-till (NT), which reduced both maize yield and biomass return during the initial 5-year transition. Nevertheless, our results indicate that, after the negative effects of transition expire, NT may increase maize yield up to the level of CT.
Our results also
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 study is the answer to the increasing worries by researchers, practitioners and farmers regarding the possibility to combine conservation tillage (i.e. NT and MT) and intensive agricultural context as that of the Po Valley. This research was dedicated to the memory of Ernesto Cervi Ciboldi. All activities were supported by the Foundation Romeo and Enrica Invernizzi (Milan, Italy). We would like to thank Cecilia Cervi Ciboldi, the farmer who hosted the field experiment and made this study
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