Balanced Nutrients

All of the papers that I’ve published have made me happy. They make me really happy when they make the transition from research, past ‘in review’ and past ‘respond to reviewer’s comments’ to the point where they are published. That is when you can actually and completely say that you are DONE. However, a few years back I co-authored a paper that made me really happy. This one was even better than usual because not only did it enter the DONE category, but the results were great. We sampled soils that had been amended with composts and biosolids over the long term and found improvements in soil physical properties, higher nutrient contents (nitrogen) and generally very high carbon storage. To me, this was definitive proof on the benefits of organics. What is even nicer, and that is the focus of this month’s library, is that other researchers continue to find the same type of response. In addition, there is a growing sophistication to the research, to both find out why this happens and how to make it better.

The first article in the library comes from Greg Evanylo at Virginia Tech. He looked at long term field sites that had a range of amendments applied at a range of rates. Amendments included poultry manure, composts, and biosolids. The biosolids were applied as compost and anaerobic cake, the latter with and without sawdust. What he found was that all amendments, with the exception of the poultry litter, increased soil carbon over the long term. The rate of increase was most rapid for the first few years after application but continued to increase for two of the three sites. He concluded that the last site had reached its holding capacity for soil organic matter. Adding sawdust to biosolids did not increase carbon sequestration over biosolids alone.

The second comes from Canada, so you can practice your French if you would like (the abstract is provided in both French and English). Here the researchers added increasing amounts of food scrap compost to a soil and measured a range of soil quality parameters 11 years after the compost was added. They all got better, particularly with the higher rates of compost addition. What also got better were yields, not every year due to variability in weather, but across all years. Compost addition resulted in cumulative yield increases ranging from 2,200 to 6,500 kg per hectare.

With these first two articles, we have confirmed the baseline. Adding organic matter to soils results in long term carbon sequestration and an improvement in soil quality that is mirrored by increased crop yields.

From here, let’s go one step up. The next paper looks at no-till practices and biosolids applications and their combination to see how carbon storage may be optimized in Virginia Coastal Plain soils. The authors sampled actual farm fields for this study. Increases in carbon stocks were seen for both biosolids-amended soils and those managed as no- till. The response was consistent across fields and soil types. If you look at the comparison between the fields without biosolids with conventional tillage and those with biosolids and no tillage, you see really significant increases in soil carbon (Table 4). This means that the combination really gets you to a soil “sweet spot” very quickly. The authors measured the silt and clay particles in the soils, noting that carbon storage increases when you have more silt and clay, as the small particles essentially protect the carbon from decomposition. They recommend no-till farming in combination with biosolids applications as a way to store maximum amounts of carbon in the soil.

The fourth paper provides even more insights in how to optimize carbon sequestration with amendments. This study was done in Oregon. The authors looked at a range of different amendments, here in combination with different cropping strategies. Amendments included biosolids, animal manures, woody waste, alfalfa and other materials. The same amount of carbon was added to each plot. However, based on the amendment used, the total N, P and sulfur added varied based on the amendment. They either grew winter wheat each year or allowed for soils to remain fallow (no crops). The found that, you guessed it, the biosolids amendments resulted in much more soil carbon storage, as did continuous wheat.

The authors concluded that the soil organic carbon (SOC) input from the plant roots were a critical factor for increasing carbon stocks. They also came up with a hypothesis as to why the carbon from the biosolids stuck around so much more efficiently than from the other amendments. What they concluded is that the N, P and S in the biosolids, the balanced nutrients in the amendment, were critical to keeping the carbon. Having the nutrients allowed for the formation of stable organic compounds which also contain N, P and S and which also persisted in the soil. Adding a high carbon material increased the soil C:N ratio for a few years, but then the added carbon mineralized and storage went back to baseline.

This finding, the balanced nutrient approach to increasing soil carbon, is the focus of the last paper as well. Here it is the greenhouse study that set the baseline for the follow up study. No biosolids were used. Rather, wheat straw amended the soil with and without the N, P, and S, and C lost from the fine soil fraction was monitored. They found that the C is stored only if you have enough N, P and S. With enough of those nutrients, sequestration is increased by up to three-fold.

So we know more now than we did when that Brown et al. paper was published in 2011. We know that soil carbon increases occurring with biosolids and compost amendments are not restricted to Washington State, but have been seen as far east as Virginia. We know that these increases are more persistent when the soils have a fine fraction to shelter the carbon. We know that biosolids and high nutrient composts are good at storing carbon because of the nutrients that they bring to the soil along with the organic matter. Finally, we know that if you combine these amendments with no-till and let the soil stay fallow as little as possible, increases in soil carbon will be even more pronounced. Even nicer than publishing a paper is finding out how to take the results from that paper and improve upon them. That is what each of these authors has done. Results are getting more and more sophisticated, but the basic message stays the same. We work with some really good stuff, so go out and use it!

Sally Brown, University of Washington