W. T. Baisden

Nitrogen inputs and outputs for New Zealand from 1990 to 2010 at national and regional scales

Abstract Reactive nitrogen (N) is increasingly added to the New Zealand environment because of increased sales of N fertilizer and increased human population. The Greenhouse Gas Inventory now reports in detail on changes for N losses from grazing animals from 1990 to 2010. Using animal numbers, we made assessments of N inputs and outputs for the 16 regions of New Zealand for 1990, 2001 and 2010 to assess temporal trends. Fertilizer sales have increased from 46 Gg N in 1990 to 329 Gg N in 2010, which leads to reduced biological N fixation by pastures. The import of oil-palm kernel has increased from zero to about 28 Gg N in 2010. Total N inputs are estimated to have increased from 689 Gg to 951 Gg N. The outputs of produce, leachate, gasses and sediment have increased from 771 to 866 Gg N; outputs to rivers may increase further if increases in outputs lag behind increases in inputs. Many of the inputs and outputs are well constrained because animal numbers have been used rather than land area, but uncertainties do exist for specific land-use classes. For example, the area of lifestyle blocks is approaching 800,000 ha and there is uncertainty regarding N inputs and outputs in this land use. There are also uncertainties in the amount of N fixation, the N loss by leaching in any one year, the amounts and fate of dissolved organic N, and the N content of eroded sediment. These uncertainties need to be resolved so that the amount of N stored in soils can be assessed. It seems likely that the N concentration of soils under dairying is increasing relative to the carbon concentration (i.e. soil C/N ratios are declining) but there is conflicting evidence as to whether the total N (and C) in these soils is increasing or decreasing.

Nitrogen inputs and outputs for New Zealand at national and regional scales: Past, present and future scenarios

Abstract The Nanjing Declaration on Nitrogen Management calls for national governments to optimise N management by several strategies including assessment of N cycles. In New Zealand, reactive N continues to be added to the environment mainly by biological N fixation, and increasingly from N fertiliser additions. Here, we extend our work on N budgets in 2001/02 for New Zealand (267 000 km2), at both national and regional scales, to 1861, 2020 and 2050. We first attempt to estimate the N cycle for 1861, the year of the first census and when European settlers were beginning to clear large areas of forest for agriculture in some regions. For the future, we adopt two scenarios: agricultural production increasing at 3% p.a., and a “cap and trade” scheme for N. These scenarios provide instructive results by projecting two very different potential policy directions into the future; they do not represent predictions. The 3% growth scenario warns of ever‐increasing N loads on the environment. The cap and trade scenario (such as may be introduced by regional councils) supports the development of a mechanism by which farmers might constrain N losses without regulations being introduced. These scenarios seek to provide farmers, industry and regulators with an understanding of the large range of future possibilities. This paper highlights the urgency with which primary industry must move away from increased production per se to systems where value is added to products.

Solubilisation of soil carbon following treatment with cow urine under laboratory conditions

There have been reported losses of soil carbon (C) under intensively grazed pastures, and soil C solubilisation following cow urine deposition was identified as a possible mechanism. We measured potential soil C solubilisation in pasture and plantation pine soils following treatment of soil with cow urine. Soils from five paired pasture and pine sites were collected. Adsorption of urine-C and desorption of soil C was determined by shaking air-dried soil with cow urine for 4 h at 4°C, decanting the urine, and then extracting the soil with water. Soil C solubilisation was the difference between adsorption of urine-C and desorption of soil C. Solubilisation of soil C in the pine soils including the organic layers was 21.6 ± 2.6 mg/g (10.5 ± 1.1% of soil C concentration), in the pine soils excluding the organic layers 7.5 ± 2.2 mg/g (18.7 ± 5.8%), and in the pasture soils 12.4 ± 5.3 mg/g (27.8 ± 7.3%). There was no significant difference with respect to soil C solubilisation between the pine (with and without organic layers) and pasture soils. Soil C lower in the profile may be as susceptible to solubilisation as C in topsoils. Adsorption of urine-C was minimal. Solubilisation of soil C under urine patches may contribute to losses of soil C under intensively grazed pastures, and this hypothesis would benefit from further testing under field conditions.

Carbon leaching from undisturbed soil cores treated with dairy cow urine

Solubilisation of soil carbon (C) under cow urine patches may lead to losses of soil C by priming or leaching. We investigated the solubilisation and bioavailability of soil C in undisturbed pasture soil treated with urine. We also studied the contribution of acid-neutralising capacity (ANC) forcing and aggregate disruption as mechanisms of soil C solubilisation. Undisturbed soil cores (0–5 cm; Typic Udivitrand) were treated with water or δ13C-enriched urine and subsequently leached. Urine deposition increased total C and dissolved organic C leaching by 8 g C m–2 compared with water. Soil C contributed 28.1 ± 0.9% of the C in the leachate from urine-treated cores (ULeachate). ANC forcing of urine was 11.8 meq L–1 and may have contributed to soil C leaching, but aggregate disruption was unlikely to have contributed. The bioavailability of organic C in ULeachate was four times greater than in both cow urine and water leachate. It is possible that ULeachate may lead to priming of soil C decomposition lower in the profile. Further testing under field conditions would determine the long-term contribution of urine deposition to dissolved organic C leaching and the fate of solubilised C in pastoral soils.

Changes in soil C, N and δ15N along three forest–pasture chronosequences in New Zealand

Changes in total soil carbon (C), nitrogen (N) and natural-abundance N isotopes (δ15N) were measured along three forest-to-pasture chronosequences on pumice soils in the Central North Island of New Zealand. On each of the three chronosequences, exotic pine forests had been converted to intensive dairy pastures 2–11 years before sampling and samples were also taken from remaining pine forests and long-term pastures (40–80 years old). The primary objective of the study was to test the hypothesis that surface-soil δ15N would increase over time following conversion of forest to pasture, due to greater N inputs and isotope-fractionating N losses (e.g. ammonia volatilisation) in pasture systems. Results supported our hypothesis, with linear regression revealing a significant (P < 0.001) positive correlation between log-transformed pasture age (log10[pasture age + 1]) and surface-soil δ15N. There was also a positive correlation (P < 0.001) between pasture age and total soil C and N, and a negative correlation of pasture age with C : N ratio. Surface-soil δ15N was also positively correlated (P < 0.001) with total soil N, and negatively correlated with C : N ratio when C : N was <13.6. These results suggested that as soils became more N-‘saturated’, isotope-fractionating N loss processes increased. Surface-soil δ15N in the pine forests was significantly less than subsoil δ15N, but there was no significant difference between the surface and subsoil in the long-term pastures, due to 15N enrichment of the surface soil. The difference in δ15N between the surface soil and subsoil may be a useful indicator of past land management, in addition to absolute δ15N values of surface soils.

Phosphorus inputs and outputs for New Zealand in 2001 at national and regional scales

Abstract Since 1990, agricultural use of land in New Zealand has intensified and some areas have received increasing loads of phosphorus (P) fertilisers. This has led to increased concentrations of animal stock units on pastures. As both stock units and human populations increase, there is increasing concern about P loss from land and towns to waters. In order to assess the extent of the issue of P loss for New Zealand (267 000 km2) we have developed a first P budget for the season 2001/02 at both national and regional council scales. The average rate of P input is estimated to be 9.9 kg/ha nationally, and this is mainly from fertiliser on pasture. The average rates of P application on pastures range from 7 kg/ha in drier regions to 28 kg/ha in regional councils with intensive dairying. The average rate of output is estimated at 4.3 kg/ha, leaving 5.6 kg/ha stored in soils; this is thought to be stabilised by both aluminium and iron compounds and within soil organic matter, where it becomes less labile with time. The region with the largest P output (17 kg/ha) is Gisborne, which has large areas of erodable land under pasture. The average P load in rivers for New Zealand is 1.6 kg/ha, and sediment‐P is the major component. Dissolved P in runoff from pastures, and effluent from animal stock also add to the P loads in waters; therefore, mitigation practices need to be introduced by land managers.

Priming of soil decomposition leads to losses of carbon in soil treated with cow urine

The extent to which priming of soil carbon (C) decomposition following treatment with cow urine leads to losses of soil C has not been fully investigated. However, this may be an important component of the carbon (C) cycle in intensively grazed pastures. Our objective was to determine soil C losses via priming in soil treated with cow urine and artificial urine. Cow urine, water, 14C-urea artificial urine, and 14C-glucose artificial urine were applied to repacked soil cores and incubated at 25°C for 84 days. We used radio-labelled artificial urine to determine the extent to which urea hydrolysis contributed to elevated carbon dioxide (CO2) emissions in urine-treated soil and as a comparison to the priming effects of cow urine. Water-soluble C, pH, dehydrogenase activity, urease activity, and CO2 evolution were monitored during the incubation. Priming of soil C decomposition (more CO2-C evolved than was added as a C source) in the cow urine treatment was 4.2 ± 0.7 mg C g–1 (5.2 ± 0.9% of soil C concentration, corrected for water control). In the cow urine treatment, ~54% of retained urea was hydrolysed and it contributed 0.4 ± 0.1 mg CO2-C g–1 to total CO2 fluxes. Low urea hydrolysis may have been due to decreased urease activity in the cow urine treatment due to the large amounts of urea present and the increased pH. Dehydrogenase activity was elevated immediately after cow urine application, and indicates that priming was likely due to heightened microbial activity. Negative priming (less CO2-C evolved than was added as a C source) was measured in the artificial urine treatments and this may reflect the differences in composition between the cow and artificial urines. Solubilisation of soil C was also found in the artificial urine treatments, but it did not appear to be correlated with increased pH or periods of greater urea hydrolysis. While cow urine decreased soil C by positively priming soil C decomposition, our artificial urine did not. Therefore, caution is recommended when using artificial urine for C-cycling research. The mechanisms by which both increased soil pH and priming occurs in urine-treated soils require further investigation.