Dissolved organic nitrogen and carbon in pastoral soils: the New Zealand experience

Dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soils are increasingly recognized as important components of nutrient cycling and biological processes in soil-plant ecosystems. The aims of this study were to: (i) quantify the pools of DON and DOC in a range of New Zealand past...

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Bibliographic Details
Published in:European journal of soil science 2007-06, Vol.58 (3), p.832-843
Main Authors: Ghani, A, Dexter, M, Carran, R.A, Theobald, P.W
Format: Article
Language:English
Subjects:
Agronomy. Soil science and plant productions
Biological and medical sciences
Earth sciences
Earth, ocean, space
Exact sciences and technology
Fundamental and applied biological sciences. Psychology
Soil science
Soils
Surficial geology
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Summary:Dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in soils are increasingly recognized as important components of nutrient cycling and biological processes in soil-plant ecosystems. The aims of this study were to: (i) quantify the pools of DON and DOC in a range of New Zealand pastoral soils; (ii) compare the effects of land use changes on these pools; and (iii) examine the seasonal variability associated with these two components of dissolved organic matter. Soil samples (0-7.5 cm depth) from 93 pastoral sites located in Northland, Waikato, Bay of Plenty and Otago/Southland, New Zealand, were collected in autumn. Adjacent sites under long-term arable cropping or native vegetation and forestry land use were also sampled at the same time to estimate the impacts of different land use on DON and DOC in these soils. Twelve dairy and 12 sheep and or beef pastures were sampled in winter, spring, summer and autumn for a 2-year period to study the seasonal fluctuations of DON and DOC. A field incubation study was also carried out in a grazed pasture to examine fluctuations in the concentrations of [graphic removed] and [graphic removed] and DON levels in soil. Other soil biological properties, such as microbial biomass-C, biomass-N and mineralizable N, were also measured. Pastoral soils contained the greatest amounts of DON (13-93 mg N kg⁻¹ soil, equivalent to 8-55 kg N ha⁻¹) and DOC (73-718 mg C kg⁻¹ soil, equivalent to 44-431 kg C ha⁻¹), followed by cropping and native vegetation and forestry soils. The DON concentration in soils was found to be more seasonally variable than DOC. There was approximately 80% fluctuation in the concentration of DON in winter from the annual mean concentration of DON, while DOC fluctuated between 23 and 28% at the dairy and the sheep and beef monitoring sites. Similar fluctuations in the concentrations of DON were also observed in the field incubation studies. These results indicate that DON is a dynamic pool of N in soils. There was a strong and significant positive correlation between DON and DOC in pastoral soils (r = 0.71, P < 0.01). There were also significant positive correlations between DON and total soil C (r = 0.59, P < 0.01), total soil N (r = 0.62, P < 0.01) and mineralizable N (r = 0.47, P < 0.01). The rather poor correlations between total soil C and N with DOC and DON, suggest other biogeochemical processes may be influencing concentrations of DOC and DON in these soils. Given the size of DON and DO
ISSN:1351-0754
1365-2389
DOI:10.1111/j.1365-2389.2006.00873.x