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

Abstract

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 NO À 3 and NH þ 4 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 ¼ 0.71, P < 0.01). There were also significant positive correlations between DON and total soil C (r ¼ 0.59, P < 0.01), total soil N (r ¼ 0.62, P < 0.01) and mineralizable N (r ¼ 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 DOC pools in the pastoral soils, we suggest that these pools of C and N should be taken into account when assessing the impact of pastoral land use on soil C and N enrichment of surface and groundwater.

Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape

“…texture or organic content play an important role on N losses. Recently, researchers have put more emphasis on studying the contribution of dissolved organic nitrogen (DON) to the N balance of landscapes (Mattson et al, 2009;Cooper et al, 2007;Ghani et al, 2007;van Kessel et al, 2009;Neff et al, 2003. Hedin et al (1995) found that organic N represents a large part of the total N losses from forest ecosystems, which was also described in other studies (van Breemen, 2002;Cooper et al, 2007;van Kessel et al, 2009;Neff et al, 2003.…”

“…Due to the same organic matter origin and similar organic components, DON is often closely linked to the C cycle (Campbell et al, 2000;Cooper et al, 2007;Ghani et al, 2007;Goodale et al, 2000;van Kessel et al, 2009;Willett et al, 2004). Accordingly, it is assumed that DON and DOC should be equally influenced by soil type or land use.…”

“…As DON includes labile and recalcitrant compounds (Neff et al, 2003) and has different retention times in soil and water, corresponding losses can substantially vary between different watersheds. Therefore, there is a need to better understand the fundamental role of DON in agricultural systems (Ghani et al, 2007;van Kessel et al, 2009) and in the global N cycle (see review by Durand et al, 2011), including its preferential flow pathways and biological degradation mechanisms (Perakis, 2002).…”

Changes in soil carbon and nitrogen pools after shifting from conventional cereal to greenhouse vegetable production

“…SMBC and SMBN were calculated as: SMBC or SMBN = (total C or N in fumigated extractsÀtotal C or N in unfumigated extracts)/K E , where K E = 0.45 (Wu et al, 1990;Jenkinson, 1988). DOC was the amount of TOC in the unfumigated extract (Cookson et al, 2007;Ghani et al, 2007) and DON was the difference between N and NH 4 -N in unfumigated extracts (Cookson et al, 2007;Ghani et al, 2007). Soil carbonate C was determined by the pressure calcimeter method (Loeppert and Suarez, 1996).…”

Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under monsoon situations

“…DON significantly contributes to N leaching losses from forest ecosystems (Campbell et al 2000;Qualls et al 2000;Perakis and Hedin 2002), and may be an important form of N losses from agricultural systems (Murphy et al 2000;McDowell 2003;Siemens and kaupenjohann 2002;Saarijarvi et al 2007). Compared to NO 3 -N, there are only a limited number of studies on DON leaching losses from agricultural soils (Ghani et al 2007;Kessel et al 2009). On average, the amount of DON leaching losses from agricultural systems was estimated to be approximately one-third of that of NO 3 -N (Kessel et al 2009).…”