Blair et al. (2005) suggested that ʻno tillʼ farming can improve the fertility and stability of soil through enhanced incorporation of organic matter and formation of macroaggregates. This farming practice minimises soil disturbance and maximises the addition of organic matter – allowing for surface organic matter to be incorporated into the soil through microbial activity and for soil aggregates to form through the activity of binding agents. Although tillage has the potential to disturb soil aggregate stability, Blair et al suggested that soil structure can nevertheless be improved using conventional farming practices if disturbance is minimised and plentiful organic matter is added to compensate for mineralisation to carbon dioxide.
Blair et al. concluded that plant residues which provide a sustained supply of soil binding agents and nutrients have the greatest potential to improve soil structure and fertility in the long term. Further research into the decomposition of various plant residues is required to understand which rotations or green manures can pro- vide the most effective nutritional releases and the most sustained contributions to soil aggregate stability.
While conventional farming practices have traditionally involved the removal of crop residues after harvesting, there can be many benefits to retaining these residues, including increased soil fertility, improved nutrient cycling and more stable soil structure. So-called ʻno tillʼ farming can also have the added environmental benefit of sequestering carbon within the soil – offsetting greenhouse gases emitted during the decomposition of plant residues.
Although there is growing acceptance that ʻno tillʼ farming can improve the structure and fertility of soil, there is still some question as to which plant residues provide the most sustained fertility and stability benefits. A recent study by Blair et al (2005), published in the Australian Journal of Soil Research seeks to shed some light on this question.
To investigate the contribution that different plant residues can make to soil fertility and stability, Blair et al took soil samples from a property near Tamworth in New South Wales. The authors investigated the breakdown of residue in two types of soil: a Black Earth comprising 32% sand, 22% silt and 46% clay, and a Red Clay comprising 29% sand, 17% silt and 54% clay. Three different plant residues (medic, rice straw and flemingia leaf) were added to the soil samples to investigate their affect on soil carbon, nitrogen and aggregate formation.
While the concentration of carbon decreased from the plant residues in all treatments, the flemingia leaf recorded the lowest decrease over time for both soil types. In just ten days, the medic residues recorded a rapid 55% and 60% decrease in carbon concentration for the Red Clay and Black Earth respectively. By way of contrast, the flemingia leaf recorded only a 15% and 22% drop. After 200 days, only 15% and 22% of carbon was retained in the medic residues for the Red Clay and Black Earth respectively, while 58% and 48% remained in the flemingia leaf. Although the rice straw was found to break down less rapidly than the medic in the first ten days of the study, over the course of the experiment, it recorded a similar breakdown rate.
Blair et al proposed that plant materials with slow decomposition rates have the greatest capacity to sustain increases in soil carbon over time and reduce mineralisation of carbon to CO2. Accordingly, the most rapid depletion in soil carbon over the course of the study was observed in the quickly decompos- ing medic treatments, while the slowest depletion of soil carbon was recorded in the flemingia treatments. Indeed, the flemingia leaf treatments were shown to provide the highest total soil carbon after a 200 day period. Blair et al (2005) concluded that, in practice, flemingia residues would provide a high rate of car- bon sequestration and slow release of nutrients for cropping activities.
It has been previously theorised (see references in Blair et al) that rapid breakdown of plant residues is predominantly dependent upon the residues having a high percentage of labile carbon in combination with high nitrogen concentrations. Although both medic and flemingia both have high labile carbon percentages and nitrogen concentrations, the Blair et al (2005) study observed very different breakdown rates for these plant residues. The authors therefore propose that other factors contribute to flemingia leafʼs slow breakdown rate: for example, its high percentage of polyphenols, tough leaf cuticles, high protein binding capacity and low dry matter diges- tability percentage. It would appear that the decomposition rate of plant residues is a more complex matter than has been previously proposed.
Unlike carbon depletion, nitrogen losses were not significant for the majority of the plant residues. In fact, loss of nitrogen was only observed in the medic samples after ten days (19% was lost from the Red Clay and 20% from the Black Earth sam- ples). After this period, no further loss of nitrogen was observed from any of the plant residues. Blair et al (2005) proposed that the initial loss of nitrogen from the medic residue was due to its rapid rate of initial decomposition, resulting in a high percentage of nitrates for denitrification. Although no significant difference in soil nitrogen was detected from the beginning to the end of the study for any of the treatments, soil nitrogen was increased compared to a control sample for all the treatments. Blair et alʼs 2005 results supports Aita et alʼs 1997 theory (European J Soil Sci 48, 283) that more carbon is lost from soil than nitrogen because internal nitrogen cycling retains nitrogen, whereas carbon is lost from the soil through mineralisation to carbon dioxide.
Aggregate stability was measured using the MWD test (Amezkeka 1999 J Sustainable Agric 14,83), which measures the size and distribution of soil aggregates. Blair et al (2005) found that soil stability was directly related to the breakdown rate of plant residues and carbon fertility. Indeed, a rapid increase in soil stability was observed for the medic treatments in the first days of the experiment, related to medicʼs initially rapid rate of breakdown. However, as the study progressed and carbon was lost from the system through mineralisation to C02, stability declined. The rice straw treatment (Black Earth) also led to an increase in MWD up to 20 days followed by a rapid decrease in stability; however, in the Red Clay, the MWD stabilised after 20 days. The flemingia treatments were the only samples to increase MWD over the entire duration of the experiment, reflecting this plantʼs slow decomposition rate. By the end of the experiment, the MWD of the flemingia treatments was nearly 80% improved in the Black Earth samples and 37%-33% improved in Red Clay samples. By way of comparison, the medic treatments were only 25%-27% improved in the Red Clay and 44% improved in the Black Earth.
The stability of the Black Earth samples improved significantly more than the Red Clay samples for all treatments in the Blair et al study. In fact, an improvement in stability was even evident in the control sample (without the addition of any plant material), in which MWD was observed to improve by 27%. These results demonstrate the capacity of Black Earth to improve in structure without the addition of organic matter, confirming Oadesʼ 1993 (Geoderma 56, 377) proposal that biological factors are less important for improving structure in clay-rich soils.
By way of contrast, the strong correlation between carbon concentration and MWD in the Red Clay samples reveals the importance of organic matter in maintaining stability in this less clay-rich soil type.
Blair et al. (2005) found higher carbon percent- ages in the outer layers of the soil aggregates from all plant residues treatments in comparison to the control. The authors proposed that newly added soil organic matter accumulates in the outer layers of aggregates, with the greatest accumulation over the 200 days occurring for the residues with a slow breakdown rate. Indeed, the flemingia treatments were found to have particularly elevated levels of carbon in the outer layers of soil aggregates. Blair et al proposed that the breakdown products of plant residues act as a binding agent – allowing macro- aggregates to form when smaller aggregates bind to old aggregates, contributing to improved soil stability. In the long-term, the authors suggest that macroaggregates reduce mineralisation of carbon to C02 by protecting soil organic matter from decom- position in an anaerobic environment and through increased contact with clay surfaces.
Blair N, Faulkner RD, Till AR and Sanchez P (2005) Decomposition of 13C and 15N labelled plant residue materials in two different soil types and its impact on soil carbon, nitrogen, aggregate stability, and aggregate formation. Australian Journal of Soil Research 43: 873-886. Read Abstract.