Sam Carrick

Random sampling of stony and non-stony soils for testing a national soil carbon monitoring system

The New Zealand Soil Carbon Monitoring System (Soil CMS) was designed, and has been used, to account for soil organic carbon change under land-use change, during New Zealand’s first Commitment Period (2008–2012) to the Kyoto Protocol. The efficacy of the Soil CMS model has been tested for assessing soil organic carbon stocks in a selected climate–land-use–soil grouping (cell). The cell selected for this test represents an area of 709 683 ha and contains soils with a high-activity clay mineralogy promoting long-term stabilisation of organic matter, and is under low-producing grassland in a dry temperate New Zealand climate. These soils have been sampled at randomly selected positions to assess total soil organic carbon stocks to 0.3 m, and to compare with the modelled value. Results show no significant difference between the field estimation (67 ± 30 Mg C/ha), the mean value of the model calibration dataset (79 ± 28 Mg C/ha), and the value predicted by the model (101 ± 41 Mg C/ha), although all estimates have large uncertainties associated with them. The model predicts national soil organic carbon stocks as a function of soil texture, clay mineralogy, land use, climate class, and a slope–rainfall erosivity product. Components of uncertainty within the model include the size and distribution of the calibration dataset, and lack of representativeness of the calibration soil samples, which were sampled for other reasons, e.g. soil survey and forest mensuration. Our study has shown that major components of uncertainty in our field estimation of soil organic carbon stocks (investigated using the indices reproducibility, RP; and coefficient of variation, CV) are short-range (within-plot) and regional (between-sites) spatial variability. Soil organic carbon stocks vary within our selected climate–land-use–soil cell due to varying stoniness (stony soils RP 44%, CV 21%; non-stony soils RP 27%, CV 13%), soil depth, slope position, and climatic effects. When one outlier soil was removed from the model calibration dataset, and the three very stony sites were removed from the randomly selected field validation set, the model calibration dataset and the field dataset means agreed well (78 ± 26 and 78 ± 21 Mg C/ha, respectively). The higher modelled value, before removal of the outlier, is likely to reflect a bias in the model dataset towards conventionally selected modal profiles containing less stony soils than those encountered by the random sampling strategy of our field campaign. Therefore, our results indicate (1) that the Soil CMS provides an adequate estimation of soil organic carbon for the selected cell, and (2) ongoing refinements are required to reduce the uncertainty of prediction.

Modelling loess landscapes for the South Island, New Zealand, based on expert knowledge

Abstract In New Zealand, occurrence of loess often determines the spatial pattern of soil depth, and influences droughtiness, leaching potential, organic matter accumulation, nutrient retention, and natural plant‐species distribution. Understanding loess distribution is therefore a major prerequisite for soil and land resource management. Although New Zealands soil scientists have accumulated a good empirical knowledge of loess distribution through several decades of field investigation, only some of this knowledge is recorded in papers and reports. This study estimates loess thickness and percent cover, and provides loess landscape models for the internal loess distribution of all land units in the South Island based on expert knowledge. We derived loess depth classes and percent cover classes and assembled land units with similar loess distribution patterns. The soil sets underpinning the map units of the New Zealand Land Resource Inventory (NZLRI) were classified according to loess depth, loess cover, and loess pattern. New loess maps of the South Island were produced from those classifications, displaying loess coverage, thickness, loess pattern, and loess landscapes. These maps present our current knowledge of the coarse‐scale loess distribution and provide a framework for fine‐scale loess landscape modelling.

Effects of irrigation intensity on preferential solute transport in a stony soil

ABSTRACT If irrigation intensity exceeds soil infiltration capacity, water may flow preferentially down cracks and large pores. In this situation, solute transport will involve only a fraction () of the soil’s water and leaching rate may be affected. To assess whether irrigation intensity affects preferential solute flow, an experiment was performed at Lincoln using 12 steel-encased lysimeters with a Lismore Stony Silt Loam soil under two irrigation intensities, 5 and 20 mm h–1. Burns’ equation was used to describe the measurements of non-reactive tracer concentration as a function of drainage. Under dry antecedent moisture conditions, bromide transport was not significantly different under the different irrigation rates, even though strong preferential leaching occurred, with of 0.23. For chloride, was 0.85 and 0.58, for 5 and 20 mm h–1 respectively, sufficient evidence to confirm the effect of irrigation intensity (P < 0.05). By assuming to be 1.00 for the median rainfall at Lincoln, an exponential function was fitted to the data, suggesting a lower limit of 0.35 for under moist conditions. Implications for nutrient leaching are discussed.

Leaching of from stony soils after effluent application.

Irrigation of dairy shed effluent (DSE) onto land is an integral part of New Zealands farming practice. The use of inappropriate soils can result in contamination of ground waters with microbes and nutrients. A gap in our knowledge is the ability of stony soils to safely treat DSE. Replicates of four stony soils were collected from the Canterbury region of New Zealand as intact soil lysimeters 460 mm in diameter and up to 750 mm deep. The soils had either stones to the surface or 300 to 600 mm fines over stones. To determine breakthrough characteristics, a pulse of DSE (25 mm depth) spiked with bromide (2000 mg L) was applied to the soil cores followed by continuous artificial rainfall, for one pore volume, at 5 mm h. Leachate aliquots were analyzed for , bromide, and NH-N. The lysimeters were then subjected to hoof pugging using a mechanical hoof, and the leaching characteristics of the soil were determined again. breakthrough curves revealed that the potential for to leach through the soils was high for Selwyn very stony soil and low for other soils analyzed. After pugging, leaching of increased in Mackenzie soil with stones to the surface. For most other soil cores, concentrations in soil leachates were low. In soils where stones are close to the surface, especially where the soil matrix is sandy, we anticipate that shallow groundwater is vulnerable to microbial contamination under some land management practices.

Testing large area lysimeter designs to measure leaching under multiple urine patches

ABSTRACT A novel, large area, repacked rectangular shipping container lysimeter (13.8 m2) was compared with those from undisturbed monolith barrel lysimeters (c. 3.1 m2), to evaluate their utility for measuring leaching under soil with multiple urine patch depositions. In July 2015, the rectangular and three barrel lysimeters had 31 and 7 patches, respectively, of synthetic cow urine applied at 3.24 g N L−1, covering 52% of the lysimeter surface area. The lysimeters then remained fallow while receiving rainfall and irrigation. Drainage and leaching dynamics were monitored until June 2016. Results indicate that the large area barrel lysimeters showed consistent dynamics in drainage, as well as bromide and nitrogen leaching. The rectangular lysimeter differed in drainage and nitrate leaching patterns compared to the barrel lysimeters. This was interpreted as reflecting changes in the pore size distribution and pore continuity that was created during the repacking of the rectangular lysimeter.

Modelling soil-water dynamics in the rootzone of structured and water-repellent soils

Abstract In modelling the hydrology of Earths critical zone, there are two major challenges. The first is to understand and model the processes of infiltration, runoff, redistribution and root-water uptake in structured soils that exhibit preferential flows through macropore networks. The other challenge is to parametrise and model the impact of ephemeral hydrophobicity of water-repellent soils. Here we have developed a soil-water model, which is based on physical principles, yet possesses simple functionality to enable easier parameterisation, so as to predict soil-water dynamics in structured soils displaying time-varying degrees of hydrophobicity. Our model, WEIRDO (Water Evapotranspiration Infiltration Redistribution Drainage runOff), has been developed in the APSIM Next Generation platform (Agricultural Production Systems sIMulation). The model operates on an hourly time-step. The repository for this open-source code is https://github.com/APSIMInitiative/ApsimX. We have carried out sensitivity tests to show how WEIRDO predicts infiltration, drainage, redistribution, transpiration and soil-water evaporation for three distinctly different soil textures displaying differing hydraulic properties. These three soils were drawn from the UNSODA (Unsaturated SOil hydraulic Database) soils database of the United States Department of Agriculture (USDA). We show how preferential flow process and hydrophobicity determine the spatio-temporal pattern of soil-water dynamics. Finally, we have validated WEIRDO by comparing its predictions against three years of soil-water content measurements made under an irrigated alfalfa (Medicago sativa L.) trial. The results provide validation of the models ability to simulate soil-water dynamics in structured soils.