Soil carbon and nitrogen biogeochemical cycles and their driving mechanisms
Carbon (C) and nitrogen (N) cycles and their coupling in terrestrial ecosystems have been a major focus of ecological research and are critical to sustainable material cycles and energy transfer as well as theoretical studies of global climate change and sustainable development. Biological factors control the coupling of C and N cycling and are also subject to feedback loops involving these elements. Research in this field focuses on C and N cycles, their coupling, and their influencing factors. By studying the mechanisms of soil C sequestration, efficient utilization of N fertilizer, interactions between soil and organisms, and C and N stoichiometry, our research seeks to elucidate the processes in C and N cycles and their driving mechanisms.
1 Soil carbon cycles and carbon sequestration mechanisms
Fate and stabilization of external organic matter
We have studied the fate of soil organic C under long-term organic fertilizer application and straw mulching. Our results have shown that the application of organic fertilizer and straw can significantly increase the organic C content in topsoil. However, twenty years of continuous cropping of corn in the Shenyang Agricultural Ecosystem Research Station (CAS) revealed that organic materials produced from internal nutrient cycling in agro-ecosystems is limited, and not sufficient to increase soil organic carbon pool in farmland. The combined application of N fertilizer and compost (N+M) is not conducive to sustaining the soil organic carbon pool (Figure 11).
Figure 11: Changes of soil organic carbon (0–20 cm) under different long-term treatments.
M: application of compost alone; the yield of compost differs in each treatment. N: nitrogen fertilizer application; NP: nitrogen and phosphorous fertilizer application; NPK: nitrogen, phosphorous and potassium fertilizer application; NPK+M: combined application of NPK and compost.
Further research found that physical protection of soil is an important factor for maintaining organic C content. We used aggregate classification and particle size fractionation technologies to test the prevailing assumption that soil organic carbon is first protected by aggregates of small particles, and then by aggregates of large particles (Figure 12). We confirmed that soil clay particles can combine more old C than sand and silts and can make an important contribution to stabilizing the soil organic C (Figure 13). Organic fertilizer application significantly increased organic C content in bulk soil for all particles. Moreover, with the increasing application of organic fertilizer, the rate of organic C accumulation increased. Accumulation of soil organic C in aggregates is consistent with our perspective that with the increasing saturation of soil organic C, external organic C first accumulates in aggregates with small-size particles, and then transfers to aggregates with large-size particles.
Transformation processes and dynamics of soil organic carbon components
Chemical components of soil organic C (e.g., carbohydrates, lignin and starch) are the fundamental factors affecting organic C stability. We analyzed the contribution of lignin, a recalcitrant organic C, to stabilize organic C with external organic C inputs. After long-term application of organic fertilizer, especially in larger amounts, phenolic monomers of lignin not only accumulated substantially in large particles of topsoil, but also persisted in small particles (Figure 14). The accumulation of lignin in clay particles occurs because the application of organic fertilizer introduced a large amount of labile carbon substrate to the topsoil, which can be used by soil microbes for their growth and metabolism, causing selective accumulation of recalcitrant lignin. In addition, the protection of soil organic C in clay particles also played a role.
Figure 14: Impact of long-term organic fertilizer application on the lignin content in each particle size. M1: low organic fertilizer input (30 Mg ha-1 a-1), M2: high organic fertilizer input (60 Mg ha-1 a-1)
Using bio-marker technology we also explored the transformation dynamics and mechanisms of organic matter input in different soil organic C pools. For example, through analyzing soil amino sugar, which is an indicator of fungi and bacteria sources, we found that fungi were a major factor of soil mineralization with applications of straw and organic fertilizer. Straw mulching and no-tillage for five years promoted the accumulation of amino sugars in the soil, and fungi gradually became the dominant component of soil flora, a process which favors the accumulation of soil organic C in farmland and enhances its stability.
We determined the contribution of plant-derived and microbe-derived C to soil stable organic C pools in different soil physical components by measuring the contents of neutral sugars. Organic fertilizer application significantly increased the neutral sugar content in bulk soil and all soil particle sizes. With increasing rates of organic fertilizer application, the rate of neutral sugar accumulation increased. Xylose and arabinose increased most significantly under organic fertilizer application since crop residues that are rich in plant-derived carbohydrates did not degrade in a rapid manner, and therefore accumulated in coarse sand particle size.
Compared to conventional tillage, conservation tillage increased the content of glomalin-related soil proteins in aggregates, and the ratio of glomalin-related soil protein to soil organic C in aggregates decreased with increasing organic C. Results indicated that the contribution of glomalin-related soil protein to the C pool was related to the size of aggregates, with larger aggregates contributing more than smaller ones. Structural Equation Modeling (SEM) revealed that soil organic C, microbial biomass, and the content of glomalin-related soil protein can explain 79% of the process of soil aggregation (Zhang et al. 2012).
Figure 15: Impact of microbe and glomalin-related soil protein on the stability of soil aggregates (microbial biomass carbon [MBC]; microbial biomass nitrogen [MBN]; soil organic carbon [SOC]; easily extractable glomalin-related soil protein [EEGRSP]; total glomalin-related soil protein [TGRSP]; and mean weight diameter [MWD])
Contribution of microbial processes to soil organic carbon
Soil organic carbon pools are strongly influenced by the catabolic and anabolic activities of microorganisms. We used an Absorbing Markov Chain (AMC) model to predict the dynamics of soil C transformations through microbial inputs and activities and found that the amount of soil microbial residues was almost 40 times that of the living biomass pool (Figure 16). Since living microbial biomass C is usually 2% of total soil organic C, the above ratio means that microbial residual C can account for as much as 80% of the total soil C. This microbe-sourced soil C will decrease under global climate change (Liang et al. 2012).
Figure 16: Contribution of soil microbial residue to the soil organic C pool
In studying the effects of soil organisms on soil aggregates and the stability of soil organic C, we found that after 10 years of applying different methods of conservation tillage, more C was stored in soil biogenic C pools, thereby causing soil organic C to accumulate and enhancing soil organic C stabilization (Zhang et al. 2012). This mechanism differs among different fractions of soil aggregates. At a size of > 1 mm, bacteria and mycorrhizal fungi were the factors influencing soil organic C accumulation, while at a size of < 1 mm, gram-positive bacteria, free-living nematodes and plant-parasitic nematodes were the factors influencing soil organic C accumulation (Zhang et al. 2013). Conservation tillage can increase soil organic C accumulation by improving nematode communities and microbial community structure and diversity in soil aggregates (Zhang et al. 2013).