In 2012 and now 2013… Retention and Best Use of Crop Residues Adds Important Soil Carbon
By Michael Petersen, Lead Agronomist for Orthman Manufacturing, Inc. January 2013
Many of us in the Conservation Tillage movement agree that as much of the past crop aftermath left on the soil surface as possible and not mixed in to the soil surface layer provides tremendous benefits both short term and long. All of us who advocate Strip-Tillage and Direct Seeding know that residues provide food for beneficial microbes, protozoa, earthworms, mites. We know that this carboniferous material improves water holding capacity and nutrient capacity – SOC (soil organic carbon) can hold up to more than 150X its weight in water. The exchange capacity to hold specific nutrients positive and negative charges is extremely high but material is mobile. SOC is vital to stabilizing soil aggregates and feeding mycorrhizal fungi which release a sticky polysaccharide compound called “glomalin” that glues soil aggregates together so they can resist raindrop impact and wind sorting. The SOC in the surface 1-3 inches resists soil crusting especially after hard rains and then baking sun.
Here in the States we have adopted better and wiser conservation measures to simplify and drastically reduce the types of pre-plant tillage which was inverting and burying prior year residues but exposing years and years of prior stored carbon and much of it was oxidizing away and above. Modelers and scientists have calculated that with practices such as Strip-Till and Direct Seeding growers can store tons of carbon in the surface 1 to 4 inches with the return and slow breakdown of crop aftermath. But what about all those roots, root exudates, lignin and cellulosic materials from 0 to 6 or 7 feet in places?
Here at Orthman Manufacturing, Inc we have observed rooting magnified SOC with the conservation tillage strategy of Strip-Tillage due to in part a quicker start in the spring to maximize the number of roots expressed below ground. We have followed that up with observed soil pits to measure corn roots in a continual strip-till program that promotes deeper expansion into the soil due to roots propensity to follow the cooler soil temperatures as the season heats deep into the soil profile. We also have gained information from studies accomplished at the University of Georgia (1999-2001) that corn is known to extend vertically at specific soil temperatures as well as its lateral root development. Gaining more root dimension in linear length we know that those of us who raise corn can accumulate more grams of dry matter below ground to depths mentioned above. That in turn adds soil carbon to the soil profile.
…SOC is vital to stabilizing soil aggregates and feeding mycorrhizal fungi….
We have measured in eastern Colorado in loam and silt loam textured very deep soils under corn; 38,100+ linear inches of corn roots with deeper rooted corn hybrids, other not quite as prolific rooted hybrids we have measured 9,500 to 20,000 linear inches. The estimate for the grams of carbon materials is far and above the grams of material remaining after the ear of corn is harvested. It is our contention as we have exposed nearly a quarter mile of roots under one corn plant that was planted into a strip-tilled environment and precision fertilized we are gaining loads more soil carbon material than what is possible with full width (inversion) tillage systems every year. To give you for example, when we first started the strip-till methodology at Lexington, Nebraska in 2007 our soil samples from the first four inches was 2.1 to 2.3%. This spring when we sampled again, in very near proximity we had 2.7-3.1% SOM. We do not graze the corn stalks with cattle which can reduce corn residues by 50% on the soil surface. We leave our standing stalks 13 to 20 inches tall and we do not shred come spring. It is our contention that the longer and taller stalks keep winter winds from blowing residues away and off into the adjoining fields or barrow ditches. We have estimated tonnage of corn residues after harvest of 5.5T/acre to 7.8T/acre and very little of it leaving the property. Maintaining this quantity of aftermath has its concerns but we have more water stored, more carbon returning into the surface, increased population of microbes, and we have old roots decomposing slowly to become humus – the treats for those burrowing insects and soil microbes.
In our long term studies at the Irrigation Research Foundation which lies just north of Yuma, Colorado, we observed from 2001 to 2008 a change in SOM from 1.51% to 2.66% in 7 years with strip-tillage. Now remember that is only being measured in the upper 10 cm (4 inches) of the soil profile. It is also important to note the elevation at Yuma is 4100+ ft compared to Lexington at approximately 2300 ft above sea level. As observed in the high plains of Colorado the residues are reduced by UV during the winter months due to the non-cloudy winter days and lack of snow cover. An accelerated oxidation of the carbon materials remaining is quite evident.
NEWS FROM DOWN UNDER…More Roots Gain SOC
Scientist Hulugalle in New South Wales, Australia has conducted some very sound measurements and reported this in Agriculture Today, April 2010 regarding what returns of carbon came from roots in two cropping systems. “We measured corn root growth in back-to-back corn and a cotton-corn rotation sown on one meter (~40inches) beds during the summers of 2007-08 and 2008-09”.
“Total carbon added from corn roots averaged 5.0T/ha/yr with cotton-corn and 9.3t/ha/year with back-to-back corn,” noted Dr. Hulugalle.
In other soil measurements Hulugalle determined that corn on corn accumulated 770 g C/m2/ yr or for those of us here in the States that is 1.7lbs of carbon per 10.7 square feet per year or 0.16lb C/sq.ft. per year. This was published in a Short Report on the Plant Root website 2010.
Much of the research and determined rates of carbon storage here in the U.S. has looked at the surface layers of the soil. Some limited research has gone into determining how much soil carbon is stored with switchgrass, a near-permanent crop and the rate is quite high. These studies are part of the biofuels industry look at how the cellulosic material can be beneficial to ethanol production and as an ecologically sound system.
RECENT EUROPEAN RESEARCH EFFORTS… More Roots = More Carbon Stored
Kätterer, T., (2011) writing in Agriculture Ecosystems and Environment observed in a review on the relative contribution of root versus shoot material to soil organic matter formation, that researcher Rasse et al. (2005) concluded that results from in situ experiments indicate that root contribution to soil organic C per unit C is, on average, 2.4 times higher than of shoots. Data ranged from a minimum of 1.5 to a maximum of 3.7, compiling results from maize, hairy vetch and alfalfa. However, in the review by Rasse et al. (2005), and studies by Barber (1979) and Plénet et al. (1993), it was assumed that the annual input of fresh root C through root exudation, turnover and cell sloughing into soil was equivalent to that of the ‘measurable root biomass’ (i.e. 100%). In the calculation by Bolinder et al. (2007), root exudation, turnover and cell sloughing was assumed to be 65% of the measurable root biomass. This is consistent with Swedish research results from a long-term (1979-88) project on agroecosystems, Ecology of arable land – the role of organisms in N cycling by Andrén et al. (1989).
The main hypothesis tested in Katterer’s research, that roots contribute relatively more to uncontrollable soil organic matter than aboveground residues, He concluded that there is strong evidence from his experiments that roots contribute more to relatively stable soil C pools than the same amount of shoot-derived plant material does.
Our work in South Central Nebraska and 16 other locations across the United States confirm that SOC is changing in the positive direction year after year with Strip-Tillage one of the best Conservation Tillage methods going. The information I shared with you provides more proof that we are helping your soil resource be sustainable, yielding well when moisture is coming from above, helps profits, reduces erosion concerns greatly with the use of Strip-Tillage.
We will be writing more and giving you more information as time marches on in 2013 as to the benefits of Strip-Tillage.