Land use and soil type affect soil C – lessons from 15 years of soil quality monitoring
The ETS (Emissions Trading Scheme) is now a reality, and C accounting is more important than ever. Accounting for all the soil C and how it changes over time is not an easy task.
The soil quality programme developed by Landcare Research in concert with Regional Councils has information on 750 sites, and some have been monitored two or three times over the last 15 years. Seven soil quality indicators relating to soil physical, chemical and biological status – total C being one of these – have been measured. Although only the top 10 cm was sampled, these data provide valuable information on how C differs with land use and soil type.
Dairy pastoral soils have the largest average C stocks followed by indigenous forest (Figure 1), although page 10 explains that soil C can be lost under dairying with time. Dairy pastoral soils also have large N stocks (note the difference in N between dairy and indigenous forest). Cropping soils have the least amount of C in the topsoil as cultivation breaks up the aggregate structure that helps protect organic C, and erosion on bare fields can wash soil C away.
Soil type also matters, as the texture, mineralogy and chemistry of a soil partly determine how quickly soil C can change and recover. Allophanic Soils generally have the highest soil C and Recent Soils the least (Figure 2).
C accounting on a national basis is difficult because land use and soil type affect soil C. Spatial modelling extrapolates sampled points in particular soil/land-use combinations to similar soils, but even so, complete and accurate measurement of soil C levels across New Zealand requires considerable effort. New quicker methods of analysing soil C content in the field will help the process.
Bryan Stevenson
Figure 1. Average soil total C and total N concentrations to 10 cm depth for major land uses (number of sites sampled in parentheses). Error bars represent one standard error of the mean. Note the difference in scale for C and N (updated from Sparling and Schipper (2004)).
Figure 2. Average soil total C and total N concentrations to 10 cm depth for some soil orders (number of sites sampled in parentheses). Error bars represent one standard error of the mean. Note the difference in scale for C and N.
