Figure 26: Dynamics of glucosamine and MurN during incubation of soils amended with glucose and 15N-labelled inorganic nitrogen
o: original; N: newly-formed; A1¾glucose and ammonium added weekly; A2¾glucose and nitrate added weekly; A3¾glucose added weekly, and ammonium added every 3 weeks.
The labeled and unlabeled amino sugars were compound-specific and substrate-dependent, suggesting that they displayed different stability in soils. Concurrent with the accumulation of the 15N-labelled MurN, the unlabelled part was decomposed during the process of incubation in all treatments. When linking the dynamics of MurN with respect to substrate availability and microbial requirements, our results suggested that MurN can play dual roles in compensating for both C and N supplies during the incubations. The dynamics of GluN degradation were also very dependent upon the stoichiometric ratio between C and N. Amino sugar polymers were hypothesized to be more readily degradable when available N was deficient, but this hypothesis was not supported by the GluN dynamics in our experiment, in which an initial enhancement in GluN formation following the reduction of N input was found. In contrast, GluN polymer will be decomposed when large amounts of C are required during nutrient immobilization by microorganisms. As a result, fungal cell wall residues were easily accumulated and contributed significantly to the maintenance of soil organic matter. Nevertheless, the partial decomposition of fungal cell wall components is still possible when an intensive amount of C is required. The transformation of bacterial cell wall residues was more rapid and played an active role in compensating for C and N demand, and thus to some degree served as the ‘capacitor’ in soil organic matter cycling.
Long-term response and adaptation
Soil amino sugars are closely involved in microbial-driven SOM turnover; thus their dynamics were significantly influenced by fertilization regimes. However, the effects of long-term fertilization (i.e., at a scale of a century or more) on the dynamics of amino sugars are currently unknown, especially when linked to soil carbon and nitrogen mineralization and sequestration. At the Rothamsted Research Station, UK, there is an extensive archive of soils from long-term field experiments, making it a unique platform for monitoring the temporal changes in amino sugar profiles in soil. Inorganic NPK applications resulted in relatively less accumulation of fungal hyphae-dominated residues, due primarily to lower C inputs into soils, rendering them low in high-quality C substrates. Continuous manure application benefited microbial growth, resulting in a larger proportional increase in bacterial than fungal amino sugar accumulation. However, cessation of manure application rapidly enhanced the relative contribution of fungal residues to SOM.
Different fertilization management regimes resulted in specific temporal patterns for the four amino sugars studied. The accumulation and decomposition equilibrium of the amino sugars was found for 160 years subsequent to the application of NPK, but a significant fraction of the amino sugars were decomposed following long-term FYM addition despite their apparent accumulation in the soil. In a specific fertilization regime, the turnover rates of the amino sugars became similar to those of SOM after the rapid formation of the acclimated microbial community, and this pattern can persist for a century. However, whenever the supply of soil C and N changed asynchronously, the GluN and MurN changed more sensitively than the total SOM in adjusting the coupled C and N dynamics of microbial metabolism. In the case of inorganic N input imposed on the FYM application, the bacterial-derived MurN decomposed more rapidly than its fungal counterpart to compensate for C demand.
In conclusion, over a period of a century the dynamics of microbial residues can well reflect the time-integrated functions and feedback mechanisms of soil microorganisms in responding to changes in the status of soil carbon and essential nutrients and thereby can be used to explore the transformation mechanisms governing the cycles of SOM in response to field fertilizer management regimes.
Figure 27: Effect of fertilization on soil amino sugars and interactions of fertilization and sampling time
NPK: NPK fertilizers were applied continuously from 1852 to the present; FYM1: manure was applied from 1852 to 1871; FYM2: manure was applied continuously from 1852 to the present.
GluN: glucosamine; ManN: mannosamine; GalN: galactosamine; MurN: muramic acid
Internal cycling of organic N components
Inorganic fertilizer is one of the most important anthropogenic inputs which influences soil nutrient turnover in agricultural ecosystems. Soil organic nitrogen (SON), generally accounting for over 90% of total soil N, plays an important role in N retention and transformation (Stevenson 1982; Schulten and Schnitzer 1998). The depolymerization of the N-containing constituents, as well as the subsequent mineralization of organic nitrogen, has been closely associated with N availability (Nannipieri and Eldor 2009). However, due to diverse origins and composition of the components, mechanisms of SON immobilization and mineralization will not be properly elucidated until more attention is paid to the specific functions of the different organic fractions involved in N turnover. At the same time, as the key process involved in the maintenance, transformation and stability of soil nitrogen (N), the incorporation and allocation of fertilizer N within different soil organic N fractions during the growing season remains largely unknown. In a plant-soil system, the cycling of fertilizer N during the growing season is closely related to temporal patterns of the transformation of fertilizer N into different SON fractions. The basal fertilizer N is initially and significantly immobilized in the form of amino acids and amino sugars by biological processes, while the topdressing of fertilizer N is apt to be retained physically rather than microbially due to low carbon availability in the arable soil. The N that is physically retained can become a part of the hydrolyzable ammonium fraction and then be easily released for crop uptake when available N is deficient. A small proportion of fertilizer N is undoubtedly stabilized in the acid insoluble fraction. Therefore, the fertilizer-derived N can be divided into three pools with different availabilities. The hydrolyzable ammonium fraction can serve as a temporary pool containing readily available N to be released quickly; while the amino acids, occurring mostly in polymeric form and associated with microbial metabolites, can be considered as a transitional pool for both fertilizer N storage and crop N requirements. The acid-insoluble constituent of SON is tightly associated with fertilizer N stabilization, but the N in this stable pool is not totally inert and this pool can still supply N when soil N is otherwise quite limited. Also important is the interim shift among the three substantial N pools to maintain soil N cycling and supply in a soil-plant system. When inorganic N is abundant in the soil, the direction of shift will be from a temporary pool to a transitional one at a relatively fast rate and then, slowly and gradually, to a stable pool. However, a shift in the opposite direction can also occur when, for example, there is a deficiency of available N due to crop uptake. Among the SON fractions, the proteinous amino acids, as important constituents in both active and slow SOM pools, play a mediating role in the retention and translocation of fertilizer N, and thus their depolymerization controls the course of fertilizer N cycling in soil-plant systems.
Figure 28: Path diagram for the relationship between 15N-labeled soil organic N fractions and total residual 15N in soil. Subscript designations for 15N-labeled soil organic N fractions and the total residual 15N in soil are identified numerically as follows: (1) Hydrolyzable ammonium 15N; (2) AA-15N = amino acid 15N; (3) AS-15N = amino sugar 15N; (4) Total residual 15N in soil