Those who consider the use of port-harvest forest residuals as a feedstock for biofuel or energy production are interested to know how biomass removal affects soil temperature, water retention and nutrient levels. These effects are being measured at Long-Term Soil Productivity Sites (LTSP) throughout the U.S including the NARA LTSP site in southern Oregon. As the name implies, these are long-term studies, and a complete understanding of the relationship between biomass removal and soil productivity is developing.
For these studies to be fully relevant, an accurate baseline measurement and understanding of soil nutrient levels in varied forest soils needs to be established. Traditionally, soil samples used to evaluate nutrient supplies and carbon pools were taken at the upper 15-20 cm (6-8 inches) of soil, recent weathering measurements have detected soil nutrient levels to as much as 100 cm (~ three feet) below the surface.
In a recent peer-reviewed paper, partially funded by the USDA-NIFA through NARA, researches review prior techniques used to monitor soil nutrient levels and explore recent knowledge that supports the need to measure nutrient levels at a level equal to root depth, which for some managed forests in the Pacific Northwest can extend well below 100 cm.
Soil organic carbon
Much of the paper focuses on soil organic carbon (SOC). SOC is claimed to be the largest pool of terrestrial carbon, and tree roots play a role in putting it there. The amount of SOC in soils vary due to soil type and biological activity, but can often be accounted for at depths up to 300 cm and greater. In one study, soils sampled at 20 cm contained an average of 59 Mg C/ha, sampling at 50 cm showed 113 Mg C/ha, and sampling at 300 cm showed 202 113 Mg C/ha. The results suggest that shallow measurements for carbon can dramatically underestimate the level of available SOC to the ecosystem. And during a time when carbon stocks are being monitored due to climate change, a rethinking to how we measure SOC is suggested.
Soil processes and roots
The paper points out that roots provide water and nutrient uptake plus they help anchor the plant. Acidity due to root respiration and chemicals released by roots are shown to promote chemical weathering of mineral components deep in the soil. The fact that soil nutrients at deep levels change seasonally, suggest that tree roots uptake nutrients in deep soil when growing and that mineral weathering, root systems and soil carbon levels are linked.
Methods to measure soil nutrient levels
The “base mineral index” was used for many years, which relied on a heavy liquid called “Thoulet solution” to separate out minerals for evaluation. The “Thoulet solution”, due to its toxicity, has been replaced with either sodium polytungstate or an agua regia analysis. Measuring the amount of nutrients in a soil sample is rather straightforward using these tools. Measuring the timescale from when a nutrient is released to when it becomes available for uptake in plants has been difficult. The paper releases new data that measures the release profiles of potassium and phosphorus over time in varied soils. The results show that nutrient release times and amounts vary depending on the parent materials (i.e. rocks) at varied soil depths. These data suggest that the amount of nutrients made available through deep root actions can be dependent on the soil mineralogy, which is at times, left out of soil quality assessments.
The paper emphasizes that in order to understand the amount of soil nutrients available with an ecosystem, samples should include soils at greater depths that are currently being measured. In addition, the type of minerals in soil can affect the amount of nutrients made available and should therefor be considered when evaluating a soil’s potential for plant productivity.
The work represented in this paper provides insight to how we assess a soil’s capacity to provide, store and replenish nutrients. A fuller understanding in this area should help researchers determine the impact of biomass harvesting on soil productivity.