Making carbon crediting really work for farmers

Carbon credits for farmers. Say those four words to 10 different people, and you’ll get at least eight different reactions.  Some people love the idea of giving farmers credit for adding carbon to their soil and think it will incentivize farmers to adopt farming practices that will save the world.  Others think it’s a scam designed to make money for a few unscrupulous players who figure out how to hijack the concept while allowing polluters to keep on polluting. Then there are many view points fall in the gray area between those extremes.

 One of the biggest questions is how we can measure soil carbon consistently and effectively in order to know whether or not it has increased, and to what degree, to determine how much credit should be given.  Many people engaging in the carbon credit debate assume that this question is answered, but it’s not.  Not by a long shot.

Carbon questions

First, there is a question as to whether carbon accrual must be measured in actual units on the ground, or whether it can instead be assumed, based on scientifically derived models of soil carbon response to different farming practices.

Secondly, there are a lot of different ways to measure soil carbon.  Most involve using either some sort of optical device to take a reading of the soil where it sits (in situ), or physical collection of a soil sample to be chemically analyzed in a lab (ex situ).  And within each of these broad categories there are many different potential methods to use. Optical options span the gamut from close-range scans with a hand-held or tractor-mounted device (using, for example, near-infrared light) to satellite scans taken from outer space, using spectral band reflectance.  Similarly, the lab option includes several kinds of wet chemistry and dry combustion techniques that offer various advantages and disadvantages in terms of speed, accuracy, and cost.

The gold standard of soil carbon content determination is combustion by autosampler. These devices require a fairly large capital investment as well as continual maintenance to keep the notoriously finicky machines running.

With all these issues and options, what’s a carbon-interested farmer to do?  And how can soil science researchers, extension agents, and carbon crediting organizations help them do it?

Rodale researchers, funded with grant money from the Pennsylvania Department of Environmental Protection’s Pennsylvania Energy Development Authority (DEP-PEDA), are testing different methods of soil carbon measurement.  Our goal is to find technologies and procedures that allow researchers and farmers to measure soil carbon more quickly and inexpensively than is possible with existing technologies.

In this article, we’ll highlight some of our findings from our first year-and-a-half of work, and in a follow-up article coming this fall, we’ll outline our plans for the next phase of the grant program.

Seeking answers

To date, our research has focused on measuring potential correlations between nitrogen mineralization potential (N-min) and soil carbon (C) content, with the goal of assessing whether measurement of changes in N-min potential can be good predictors of changes in soil C.  The proposed benefit of measuring N-min potential to track changes in C is that N-mineralization rates change much more quickly than levels of C in the soil (it can take 3-5 years to register small increases or decreases).  Thus, if N-min potential correlates well with soil C levels, then the more rapid changes in N-min rates could be used to signal changes in soil C more quickly than they can be measured by soil C tests alone.

To test this hypothesis, we measured N-min potential on four groups of soil samples from 40 corn and oat plots in the Farming Systems Trial (FST) in 2008 (collected on June 17, July 30, October 16, and December 15) and have sampled the same 40 plots, now in oats and soybeans respectively, twice so far this year (again on June 17 and July 30). 

We began by using soil probes to take 15, 8-inch deep soil samples from different points all over each FST plot (which are all 20 feet wide and 300 feet long), and then pooled and mixed those samples in a plastic bag to create a representative soil sample for that plot. These samples were kept cool and at natural moisture levels until the next day, when we took them into the lab and sieved them through a 2 mm sieve. 

At that point, the soil was adequately prepped for analysis of the sieved soil’s total C and N content by combustion autosampler. Penn State’s Agricultural Analytical Laboratory performed the determination on one of these devices housed there.

Meanwhile, back at Rodale we worked on determining the soil’s nitrogen mineralization potential, a metric that simultaneously assesses the soil’s biological and chemical characteristics. The determination method requires a labor-intensive, lab-based incubation and extraction technique.

Sieve, shake, extract

We took a portion of the sieved soil sample and mixed it with a potassium chloride (KCl) solution in a centrifuge tube, and then shook it for an hour.  The purpose of this work was to extract any existing mineralized N from the soil into the liquid.  After the hour of shaking, we put the sample in a centrifuge for 10 minutes, and then extracted off some of the liquid to be sent to the Soil and Plant Nutrient Laboratory at Michigan State University for analysis of ammonium (NH4) and nitrate (NO3), which are the outputs of the N-min process.  We called this extraction Time 0 (T0), representing a “baseline” of mineralized N in the soil at the beginning of a given time period.

Another portion of the sieved soil sample was placed into 10ml of deionized water in centrifuge tubes. Any air in the tubes was removed and replaced with N2 gas.  We then put the tubes in a 30o C (86o F) incubator for a full week, to allow microbes in the soil to continue mineralizing N for that time period.  After the week-long incubation, we then added KCl to the soil/water mix to extract the mineralized N from the soil, and then shook, centrifuged and extracted the liquid from the samples for ammonium analysis, as with the T0 samples.  This incubated group of samples was called T1. The difference in mineralized N content between the T0 and T1 samples is the “N-min potential,” or the amount of organic N the microbes were able to turn into inorganic (plant available) N in one week.

With data in hand, we charted the N-min potential (ug NH4-N/g soil/day) with the soil C content for each sampling date.  Here we found that the N min and C data correlated very well (R2 = 0.72) when the data from all four of the 2008 sampling dates were averaged for each of the six FST farming systems. These FST systems are: conventional tilled, conventional no-till, organic manure tilled, organic manure no-till, organic legume tilled and organic legume no-till. 

However, when we charted these two variables for each sampling date, we found that N-min potential varied widely over the course of the year, while soil C levels didn’t change much at all, suggesting that changes in N-min potential do not correlate well with changes in soil C, at least not in short term sampling. 

Organic more N-active

However, we did find very strong correlations between farming practices and N-min potential.  Data showed that N-min potential was highest in the organic systems, and low in the conventional systems, suggesting that the organic systems support greater soil microbial activity, which in turn, provides a long-term, reliable supply of N to plants throughout the year. 

Conventional systems that receive chemical fertilizer and few organic matter inputs provide a less hospitable environment (fewer food sources and less stable soil moisture levels) for N-mineralizing microbes than organic systems that receive organic matter from compost and/or cover crop inputs.  This information is useful to farmers, confirming that increasing soil organic matter content really does improve N cycling, with potential to increase plant nutrition.

We are repeating these analyses this year to assess the validity of these results, since agronomic research requires several years of data to confirm trends. We need to compensate for annual variations in factors such as weather, crop establishment and crop growth, as well as natural variability in soils.  However, based on the first year of data, we believe that N-mineralization potential will not prove to be a reliable or cost-effective means to predict changes in soil C.

As such, we are currently reviewing a number of different field instruments that measure soil C directly in the field, without the need to send samples to a lab, and we plan to purchase one of these instruments for field testing in the coming year. 

A practical, on-farm testing system that would accurately track year-to-year soil carbon variations will be key to building confidence at the agricultural end of a carbon-offset program. Many other factors will be involved in how documented d soil carbon changes could be used in a larger climate change mitigation plan.

Our next article in this two-part series will describe some of the different instruments we’re looking at, and how we settled on the system we’ve chosen to purchase and test.

Christine Ziegler Ulsh is science editor and a research technician at the Rodale Institute.

This research was supported with funding by the Pennsylvania Department of Environmental Protection’s Pennsylvania Energy Development Authority (DEP-PEDA), grant number 41000455440, “Rapid, Cost-effective Soil Measurements for Accurate Agricultural Carbon Crediting.”