The Quantitative Soil Pit Method for Measuring Belowground Carbon and Nitrogen Stocks

Abstract

Many important questions in ecosystem science require estimates of stocks of soil carbon and nutrients. Quantitative soil pits provide direct measurements of total soil mass and elemental content in depth-based samples representative of large volumes, bypassing potential errors associated with independently measuring soil bulk density, rock volume, and elemental concentrations. The method also allows relatively unbiased sampling of other belowground carbon and nutrient stocks, including roots, coarse organic fragments, and rocks. We present a comprehensive methodology for sampling these pools with quantitative pits and assess their accuracy, precision, effort, and sampling intensity as compared to other methods. At 14 forested sites in New Hampshire, non-soil belowground pools (which other methods may omit, double-count, or undercount) accounted for upwards of 25% of total belowground N and C stocks: coarse material accounted for 4% and 1% of C and N in the O horizon; roots were 11% and 4% of C and N in the O horizon and 10% and 3% of C and N in the B horizon; and soil adhering to rocks represented 5% of total B-horizon C and N. The top 50 cm of the C horizon contained the equivalent of 17% of B-horizon C and N. Sampling procedures should be carefully designed to avoid treating these important pools inconsistently. Quantitative soil pits have fewer sources of systematic error than coring methods; the main disadvantage is that because they are time-consuming and create a larger zone of disturbance, fewer observations can be made than with cores.

Soil phosphorus supply controls P nutrition strategies of beech forest ecosystems in Central Europe

“…For the other soil analyses, we used soil samples derived from volume based sampling of a complete soil profile performed over an area of 0.25-0.56 m 2 , down to 90-100 cm below the mineral soil surface. Due to the high stone contents of the soils, we decided to use the ''quantitative soil pit'' (QP) approach developed by Hamburg (1984) and adjusted recently by Vadeboncoeur et al (2012) to quantify carbon (C) stocks in stony forest soils. The method provides volume-based samples of fine earth material and other constituents, such as roots and stones, of a soil pit.…”

Carbon fluxes and interannual drivers in a temperate forest ecosystem assessed through comparison of top-down and bottom-up approaches

“…Here we focus on changes in woody carbon stocks and assume that changes in soil carbon stocks were minimal as was found from measurements at mature stands in nearby Hubbard Brook Experimental Forest (Yanai et al, 2013). Changes in soil carbon stocks would be very difficult to detect over a 13 year study period (Vadeboncoeur et al, 2012). Instead, in these mature (100-125 year old) stands we assumed that there was little to no net change in annual soil C stocks; however, we included an uncertainty of ± 40 g C m −2 yr −1 (Post and Kwon, 2000).…”

“…belowground biomass production), which can introduce substantial uncertainties (Clark et al, 2001) Estimating net C exchange from changes in major C stocks offer yet another approach, the benefits of which include its straightforward nature and lack of reliance on difficult-tomeasure fluxes. However, belowground C pools are large and notoriously variable, making change detection extremely difficult (Vadeboncoeur et al, 2012). And, on its own, this method doesn't offer insight into mechanisms or subcomponent C fluxes.…”

Review of Soil Organic Carbon Measurement Protocols: A US and Brazil Comparison and Recommendation

“…Methods to overcome sampling-induced compaction have been developed, but other potential sources of inaccuracy persist. For example, underestimating of the coarse soil fractions (>2 mm) can occur if the coring equipment is unable to collect larger rocks [109]. Fortunately, for most agricultural soils with low rock content, this effect is probably negligible.…”

“…Soil pits can also eliminate the accuracy constraint of cores by allowing a more specific measurement of soil mass for bulk density assessment [109]. The major problem associated with pit sampling is site disturbance, and thus the diminished possibility for re-sampling at the same location (i.e., diachronic research).…”