Long-term effects of crop rotation, manure and mineral fertilisation on carbon sequestration and soil fertility
Introduction
Sustainable agro-ecosystem relies upon an adequate amount of soil organic matter (SOM) (Paustian et al., 1997). C sequestration in the soil reduces the atmospheric content of carbon dioxide, which is considered one of the major greenhouse gases (Lal, 2004a, Lal, 2004b). Good cropping techniques can favour the soil organic carbon (SOC) build up in the soil (Powlson et al., 2011). For example, many authors reported significant SOC increase in manure added fields compared to soils receiving only mineral fertilizers (Christensen, 1988, Gerzabeck et al., 2001, Kenneth et al., 2007). Manure distribution not only directly supplies organic C to the soil, but it also favours soil aggregate stability, due to the binding action of humic substances and other microbial by-products (Smith et al., 1997, Haynes and Naidu, 1998, Whalen and Chang, 2002). This stability helps C accumulation because the organic matter is physically protected from microbial attack within soil aggregates (Campbell et al., 2001). In many experiments the distribution of mineral fertilizers on manure amended soils significantly increased C sequestration. This effect can be ascribed to the higher production in well fertilized soil, that increases the quantity of crop residues that are left on the soil surface and are buried by ploughing. However, without manure, even a high mineral fertilizer rate could not modify SOC content (Hao et al., 2002, Rudrappa et al., 2005).
Many authors also found that crop species and their sequence play a major role in soil C retention (Reeves, 1997, Potter et al., 1998, Wright and Hons, 2005a, Wright and Hons, 2005b, Morari et al., 2006, Varvel, 2006; among the others). In particular, C sequestration was reported to be greater under rotations of various species than under cropping systems in which the same type of crop was grown continuously (Franzluebbers et al., 1994, Franzluebbers et al., 1995). In the short term the amount and type of residues that are left in the field appeared crucial (Lynch and Bragg, 1985, Xu and Juma, 1993, Martens, 2000, Follett, 2001, Lorenz and Lal, 2005). The decomposition rate of plant debris was found to be mainly governed by their C/N ratio. With time the lignin content becomes the limiting factor of SOM mineralization (Tian et al., 1992).
SOM can hardly be modified because complex buffering processes increase the system resilience. The burial of organic materials improves the soil physical characteristics; better soil conditions promote the microbial activity (priming effect) and, consequently, speed up SOM degradation (Kuzyakov et al., 2000). Thus any adjustment of the organic matter in the soil is slow, and the influence of agronomic techniques on SOC dynamic can be studied only through long lasting field experiments (Gerzabeck et al., 2001). Most researches with this aim analysed soils that were sampled only in a certain year after the start of the experiment; but SOM can markedly vary according to weather conditions and possible trends over time can be masked by a wide annual variability. Thus studies on SOC require a set of assessments over many years.
Our experiment was aimed at the evaluation of the interactions between type of crop rotation, manure and mineral fertilisations on the dynamics of organic matter and nitrogen in the soil. We wanted to demonstrate that a suitable combination of crop rotation, mineral and organic fertilization can significantly increase the content of stable organic matter and nitrogen in the soil. Consequently it can increase soil fertility and C sequestration in tilled soils. Discussed data have been obtained from a long-term field experiment in the fertile Italian Po valley, near Bologna.
Section snippets
Field experiment
A long-term field experiment was started in 1967 at the Experimental and Didactic Farm of Bologna University, located in a plain near Cadriano (44°33 N, 11°21 E; 32 m a.s.l.), in the southest Po valley (Italy). The soil is an alluvial silty loam, classified as Udic Ustochrepts fine silty, mixed mesic (Soil Taxonomy) or Haplic Calcisol (FAO-UNESCO). At the trial start it had the following characteristics (on a dry matter basis): 58% sand (ø 2–0.02 mm), 15% silt (ø 0.02–0.002 mm), 27% clay (ø < 0.002
Soil organic carbon (SOC)
During the 36 years of the experiment, both crop rotation and fertilisation influenced the organic C concentration in the top 0.40 m of soil, without significant interactions between them.
In the first 18 years SOC decreased in all plots, according to exponential trends (Fig. 2, Tables 2 and 3 ), that imply steady percentages of annual loss.
Rotation significantly influenced the degradation rate (Fig. 2 and Table 2). Averaged over all fertilisations, SOC depletion was slow under continuous wheat
Discussion
The initial decline of SOC content was due to a rapid demolition of the previous organic matter caused by the changes in the agronomic management. The experiment drastically modified: soil tillage, which became deeper and more frequent than before, residue management, that started to be yearly removed, and crop rotations, most of which were simpler than before, particularly when alfalfa meadow was abandoned (the typical poliannual forage crop in that time). Similar exponential organic C
Conclusion
As found in other long-term experiments, the intensification of the agricultural system, concerning ploughing depth and catch crops cultivation, rapidly decreases SOC content. Both the inorganic N fertilisation and manure supply can increase the SOC level in the soil. From an environmental point of view manure recycle within a farm appears the best way to sequestrate atmospheric CO2 in the soil. Anyway, C losses through cattle respiration and fermentation processes during manure maturation
Acknowledgements
In the last years the Italian University and Research Ministry (within “PRIN” and Biosus Projects), and the CARISBO Bank Foundation supported the research.
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