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Inorganic nitrogen ( IN) release kinetics and exchangeable inorganic nitrogen ( EIN) of the sediments from shallow lakes in the middle and lower reaches of the Yangtze River region were investigated. The results showed that the EIN contents of the studied sediments ranged from 131 to 274 mg/kg. With the increase of the total nitrogen ( TN) contents, their EIN also significantly increased. Their ratios of EIN to TN ranged from 5 to 17%, and with the increase of their TN contents, their ratios decreased. The relative contribution of NH4 +-N and NO3-N to EIN ranged from 83 and 11% to 89 and 17%, respectively. The curves of the NO3-N release kinetics of the studied sediments were not completely consistent with those of IN and NH4+-N. The amounts of the IN released from sediments not only relate to their EIN contents, but also to other N fractions. [ABSTRACT FROM AUTHOR]

The contents of different organic nitrogen ( N) fractions, their contributions to total N ( TN) and the relationship between their mineralisable N ( MN) and different organic N fractions were investigated. Acid hydrophsable N ( AHN) and nonhydrolysable N ( NHN) accounted for 70.66 to 91.69% and 8.31 to 29.34% of TN. The AHN was the main organic N fraction, and its relative contribution to TN was higher than that in soil. So in terms of AHN contents and their contribution to TN, sediments are more hazardous than soils for lake eutrophication in the studied area. The ammonium N ( AN), amino-acid N ( AAN), amino-sugar N ( ASN) and hydrolysable unidentified N ( HUN) accounted for 30.51, 31.38, 6.42 and 31.69% of the total organic N, respectively. AN and AAN were the main sources of the MN in the studied sediments, which contributed to N release more than that of ASN and HUN. [ABSTRACT FROM AUTHOR]

Abstract: The contents of different organic matter components and dissolved organic matter (DOM) release kinetics of the sediments from the middle and lower reaches of the Yangtze River region were investigated, and their relationships discussed. The results show that organic C (OC) ranged from 8.14 to 43.65gkg−1, dissolved organic C (DOC) from 0.38 to 1.38gkg−1, active organic C (AOC) from 1.12 to 4.45gkg−1, heavy fraction organic C (HFOC) from 6.86 to 39.08gkg−1, accounting for 2.42–9.34%, 8.66–29.72% and 84.29–93.18% of OC, respectively. With increasing of OC content the ratios of DOC to OC and AOC to OC decreased. The contents of AOC, DOC, light fraction organic C (LFOC) and their contribution ratios to OC in studied sediments were higher than those reported in soils. The DOM release process of the studied sediments includes rapid and slow stages, and the rapid release occurred within 30min, mainly in 5min. The DOM release kinetic data in this investigation can be best fitted by the Power Function model. The correlations between total N (TN), total P (TP), OC, DOC, AOC, LFOC, HFOC and the DOM release kinetic parameters (k, c, a, b, rate 30) of the sediments were significant. There were also significant correlations between TN, TP, OC, DOC, LFOC and HFOC in sediments. So the DOM release from sediment was not only related to the OC content, but also related to the organic matter composition characteristics, especially the contents of DOC, AOC and LFOC. [Copyright &y& Elsevier]

Abstract: Potentially mineralizable N (PMN) in sediments is an important N source to aquatic ecosystems. However, there are few studies on PMN in lake sediments. The main N fraction in lake sediments is organic, and mineralization is through benthic metabolism in sediments rather than in the water column in most shallow lake systems. It is shown that PMN contents in the studied sediments were higher than the inorganic N in the shallow lakes in the middle and lower reaches of the Yangtze River area, China, and were also higher than those in soils from the same region. PMN constituted 3–26% (14%, on the average) of total N (TN) in sediments. Amino-acid N was the major contributor to the organic N pool in PMN. Potentially mineralizable N, TN, total P, organic matter, cation exchange capacity and fine-grain size were significantly correlated. With TN increasing in the slightly polluted sediments, PMN increased more rapidly in the heavily polluted sediments. The results suggest that more attention should be paid to organic N mineralization both in heavily and in less polluted sediments. [Copyright &y& Elsevier]