Amino acids are one of the largest identifiable groups of organic N compounds in soils

Amino acids are one of the largest identifiable groups of organic N compounds in soils (Hopkins 1996). D-amino acids are far less abundant in nature than L-amino acids. Both L- and D-amino acids enter soil from different sources including plant, animal and microbial biomass, antibiotics, faeces and synthetic insecticides (Hopkins, O’Dowd et al. 1997; Andersson and Berggren 2005). Moreover, D-amino acids appear in soil due to abiotic or biotic racemization of L-amino acids; degradation (bacterially mediated) of D-amino acids is slower than L-amino acid (Hopkins 1996; Hopkins, O’Dowd et al. 1997). Both L- and D-amino acids occur as bound in soil organic matter and as “free” amino acids dissolved in soil solution or exchangeably bound to soil colloids (Andersson and Berggren 2005). Plants have a capacity to directly take up D-amino acids by their roots but they can utilize small quantity of D-amino acids and it inhibit plant growth (Hopkins 1996; Hopkins, O’Dowd et al. 1997; Andersson and Berggren 2005). For consequent experimental determination of their significance for crop production and growth of plants in different types of managed and unmanaged ecosystems (Hopkins 1996; Hopkins, O’Dowd et al. 1997). Furthermore, microbial death also provides free amino acids that plants are able to take up (Campbell 2010). Physical processes such as sorption of amino acids to the soil matrix play an important role in free amino acids (FAA) composition and strongly affect the amino acid composition in sandy, acidic forest soils (Perez, Zhang et al. 2015). Moreover, Perez, Zhang et al. (2015)) found that organic soils contained higher concentrations of soil FAA (such as histidine and lysine). However, the addition of urea (organic fertilization) altered the composition of several soil FAA in organic soil. Glutamic acid was the only amino acid to increase significantly in proportion after addition of straw (Perez, Zhang et al. 2015). In addition, a high concentration of glutamic acid was found to correlate with the organic matter content (Perez, Zhang et al. 2015). Similarly, the L-to- D ratios for glutamine and glutamic acid could be an indicator of acid-stress at the community level (Hopkins, O’Dowd et al. 1997). Tryptophan is neutral and non-polar and thus would not be preferentially sorbed to the solid phase, though it has been reported that tryptophan is abundant in soil (Perez, Zhang et al. 2015). Serine was also one of the most abundant amino acids in forest and agricultural soils which can undergo abundance shifts within weeks (Perez, Zhang et al. 2015). Most of the accumualted proline in plants was transferred without metabolic conversion, though some conversion to glutamine and alanine occurred (Schobert, Köckenberger et al. 1988). It was also suggested that roots successfully compete with microorganisms for free amino acids in the soil (Schobert, Köckenberger et al. 1988). P. glandulosa and woody cluster soils had higher amounts of valine, threonine, and proline serine, hydroxyl-proline, and polar amino acids (Creamer, Filley et al. 2013). Lysine, which is higher in grassland soils, is present in dicot extensins, although in monocot extensins it is in higher concentrations as it is often substituted for hydroxyproline (Creamer, Filley et al. 2013).
In this experiment various combinations of organic and inorganic fertilizers input for their residual impact on biochemical, biological and physic-chemical attributes of arable field was assessed. Table 1 exhibit that highest SOC contents under the OM (12.20 C kg-?1) followed by HOM (9.28 g C kg?1) treatment with significant (P ? 0.05) difference than control. Contents of total N in soil were statistically (P ? 0.05) higher with the OM (1.56 g N kg?1) and HOM (1.10 g N kg?1) treatments than NPK (0.80 g N kg?1), NP, NK, PK and CK. These results are in lined with the work of many researcher (Hopkins and Shiel, 1996; Malhi, Wang et al. (2005) who observed that relative to the control, biomass C and other nutrients (D-amino acids) had been increased by FYM and LFOM, and reduced by NPK treatment. Moreover, the presence of C and N contents in soil was attributed to the amount of crop residues, which in turn was associated with crop yield (Malhi, Wang et al. 2005). N-fertilizer application could decrease the acidic amino acids and increase neutral amino acid concentration in soil (Malhi, Wang et al. 2005). This strongly suggests that acidic amino acids are a source for N mineralization and that neutral amino acids represent storage pools for soil organic N and C (Malhi, Wang et al. 2005). Malhi, Wang et al. (2005) suggest that proper N fertilization may be an important consideration, as amino acids in soil increased considerably with N application (Malhi, Wang et al. 2005). Peroxidase could also be considered to enhance soil C and N mineralization via improving the bioavailability of reducing sugars and amino acids (Malhi, Wang et al. 2005).
The pH values were slightly lower than that of original soil (8.65), being highest in control (8.61) followed by NK (8.58) and PK (8.35) differing non significantly. The lowest pH was found under OM and NP treatments with a value of 8.01. Lowest pH of soil may be considered as favorable for accumulation of acidic to neutral amino acids.
Mass proportions of macroaggregates (M) were significantly (P ? 0.05) enhanced with manure amendments as 30.8% under HOM and 17.7% through NPK, while the lowest percentage (8.8%) was obtained from OM and NP. However, the proportions of microaggregates (m) were increased in the OM and NP treatments as compared to control and other treatments, showing results just the opposite to that of macroaggregates. Thus treatments of OM (71.5%) and inorganic fertilizer NP (70.1%) showed the highest mass proportion of microaggregates. Free silt + clay (SC) portion reflected significant reduction with HOM (14.1%) and NPK (19.4%)
The soils amended with OM resulted in a statistical increase (P ? 0.05) in L-aspartic acid followed L-alanine and L-lysine as compared to other amino acid and all other treatments as well as control. This was in a pace with that of Andersson and Berggren (2005) who studied that the dominating amino acids during their investigation were glycine, glutamine/histidine, aspargine/serine and aspartic acid other than arginine. So OM was found to be a best treatment among all other treatments. Which may be supported by the finding of Malhi, Wang et al. (2005) who consider OM as an important factor to sustain and improve soil quality and productivity. Similarly, our results also indicated that OM followed by HOM treatment was found to be the most effective among all other treatment for enhancing the D-amino acid contents. This was in contrast to that of Brodowski, Amelung et al. (2005) who suggested that the amount of D-amino acids can be declined bi-exponentially to about 30% with increasing number of years for cropping. As the years of arable cropping increased, the proportions of D-alanine and D-glutamic acid increased relative to the respective L-enantiomers. In contrast, the D=L-ratios of leucine and aspartic acid declined in the long-term cultivated plots, probably reflecting losses of old amino acid-N reserves at the most degraded arable land (Brodowski, Amelung et al. 2005).

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