Grazing Land Management and Soil Carbon: A Climate Hubs – LTAR Webinar Series

Did you evaluate fertilization effect in?

Fertilization was not a treatment in this design. Other studies have investigated fertilization with mixed results. These results could be summarized for grasslands if needed.

Did you evaluate fertilization effect in SOC or TSN?

Fertilization was not a treatment in this design. Other studies have investigated fertilization with mixed results. These results could be summarized for grasslands if needed.

I have seen fields full of weeds with muddy soils and the rancher claimed to be using rotational grazing. How can rotational grazing be called “improved”. Are we missing something?

Indeed, rotational stocking by itself is not necessarily an improved practice when appropriate stocking rate is not considered first. Type of forage and condition of the sward from past management should be considered so that an improved trajectory can be developed.

Was grassland grazed or hayed or both?

Grasslands in this study were a combination of grazed pastures, haylands, and mowed fields.

Do you have any data about differences among different grasslands? Specifically, it would be interesting to see differences between pastures that have cool season grasses such as tall fescue and pastures with warm season perennial grasses.

Yes, there are some older studies in the region that have this type of forage growth habit (see Franzluebbers et al., 2000; Soil Biology & Biochemisty 32:469-478). There have been some newer samples collected from a study in North Carolina that will also help to answer this question, although it will be largely confounded by soil textural differences in two contrasting regions of the state.

How much N fertilizer is used in the pastures and i the crops (conventional and no-till)?

The history of fertilization on all land uses was a bit fuzzy, but typically pastures would have had minimal fertilization (<40 lb N/acre/year) while conventional-till and no-till crops would have had larger N inputs of approximately 100 lb N/acre/year depending on the type of crop and its frequency in the rotation.

How were the pastures managed in the study? Was it grazed by just cattle?

Although not all grasslands in this study were grazed, those that were grazed were mostly by beef cattle.

How do you consider the initial carbon content when assessing the effects of land use and management on soil organic carbon accumulation?

The root-zone enrichment calculation considers the baseline condition of a land use as the carbon or nitrogen concentration at 30-cm depth (12 inch depth). Typically, a baseline condition has been at a point in time with measurement of change over time as sequestration. However, the baseline concentration at 30-cm depth is replacing the temporal baseline condition because there is an assumption that this concentration has not changed (or will not change substantially with several decades of management). However, the baseline concentration helps to give a more precise baseline condition than a simple average for typical soils in the region.

How did “improved grazing” affect the number of livestock kept or produced on a given area of land overall?

This study didn’t specifically focus on different levels of grazing management. It was only focused on land use. There were too few locations with grazing of forages to make a good evaluation of improved vs unimproved grazing management.

Also, how variable were “improved” grazing characteristics? How did you ensure consistency in the survey?”

The 12 inch depth was chosen based on a few criteria – (1) typically soil C is reported as 0-30 cm depth (0-12 inches), so the bottom end of this increment was considered important, (2) the plow layer is typically considered not to exceed 12 inches, so this would be below the depth of tillage influence, (3) several studies show that the vast majority of root mass is within the surface 12 inches, (4) the depth was intended to be deep enough to not be influenced by management but also not too deep to be relevant to the surface soil conditions. One thought might be to consider a 40 cm depth (16 inches), but so far the 12 inch depth seems reasonable in the soil profiles analyzed thus far.

Very informative presentation Alan. Is the “no-till” cropland treatment actual no-till (or infrequently tilled) and for how long?

The no-till croplands that were sampled were for variable duration, but were continuous no-till. Some of the NT fields were for 35 years of duration, but some were only for 5 years of duration. This is a limitation of the study.

What would you say to someone using this research to support land conversion from grassland to woodland to increase carbon capture?

The data collected indicated some increase in soil organic C with woodland compared with grassland, but the effect was not as large as with other land uses. There would have be another reason to make conversion of grassland to woodland. A combination of grassland and woodland as a silvopasture could be a good compromise if this were desired by the landowner.

What are your thoughts on the potential effect of atmospheric CO2 enrichment and its effects on plant production on the observed trend of increasing soil carbon over time?

The effect of increasing atmospheric CO2 concentration on soil organic C accumulation is an interesting thought and this study has no direct evidence to support or refute this effect. However, based on other studies it is very possible that there was some contribution of increasing atmospheric CO2 concentration on some of the fields. Theoretically this should have occurred more in C4 dominated land uses and some of the grasslands and croplands would have been more affected than C3 dominated land uses (most exclusively woodlands).

When should soil samples be collected for analysis of biological activity if the goal is to fertilize in the fall? Would it be different if the fertilization occurs in spring?

Soil sampling should occur prior to or early in the growing season of interest. If the growing season is the autumn/winter, then sampling at the end of summer prior to fall growth would be preferable. If the growing season of interest is the spring, then sampling soil in early spring or late winter would be appropriate.

Since grazing management varies so widely, could a functional indicator for improved grazing lands be grazing stand health (plant type, variety, height)?

Yes, I agree that some of these plant health indictors (plant type, variety, height) could possibly be important signals of how much soil organic C and N could increase in grassland settings. These variables just need to be tested and validated to make those claims.

How are carbon sequestration and nitrogen sequestration related? Is it just signifying an increase in biomass? What is the reason to measure the nitrogen sequestration?

Carbon and nitrogen are bound together in soil organic matter. Generally about 58% of soil organic matter is carbon and about 5% of soil organic matter is nitrogen. When carbon is sequestered as part of the conservation management approach, nitrogen is also being sequestered in soil organic matter. Nitrogen can be a more relatable element to landowners since nitrogen fertilizer input is costly and needed by growing forages.

Do you think there is an influence of prescribed fire in these different land uses? For example, are the woodlands burned? Thank you.

Prescribed fire could have an impact on soil organic C and N but this was not specifically investigated in this set of data. A few of the woodlands had been burned once or twice in the past, but most were not typically burned.

These findings are from an ultisol soil; would you expect other soil types to also have ~10 years of steep improvement before shifting to marginal gains?

Yes, I think that the temporal projections are likely to be similar in other soil types. However, the antecedent condition of a field prior to establishing a grassland could have a large effect on the magnitude of response in soil organic C and N. These types of experiments are possible simply by sampling a diversity of grasslands in a region and calculating root-zone enrichments to overcome the variability caused by different soil types, textures, and historical management conditions.

Did you see any evidence of movement of SOC across depths in the no till practice lands?

There is clear evidence for soil organic C accumulation in the top 10 cm and sometimes in the top 30 cm, but there is little solid evidence from this study and others too of significant soil organic C and N accumulation due to management below 30 cm. It should be clear that there is soil organic C and N below 30 cm, but the evidence that they are management-induced remains highly unlikely. These accumulations were much more likely as part of the pedogenic process that occurred thousands of years in the making.

The results in woodlands, can they be affected by woodland litter? It is usually very hard to separate litter and that is higher in C:N ratio

Surface litter under all land uses was collected and analyzed for C and N contents. With this in mind, surface litter was pushed to the side during soil sampling so as to only sample the beginning of mineral soil under woodlands and in all land uses.

It would be interesting to take these measurements in a silvopasture setting.

Indeed, a silvopasture setting could reveal unique data. There was only one research station location that had a silvopasture setting but it had a duration of only 8 years, so duration would have been a limitation compared with all other longer term land uses.

What are the next steps for your research in this area?

The next steps of research are to collect and analyze root-zone enrichment under a diversity of land uses on private farms throughout the southeastern US. This has begun with samplings in Virginia, Kentucky, and North Carolina under grasslands, as well as in Georgia, South Carolina, and North Carolina under croplands. Further expansion into other states is expected in the next few years.

Any similar studies happening near WY? How applicable are your results out West?

Climatic conditions should have a large impact on soil organic C and N under long-term land uses. I would expect some different results in locations in the west. It might be more difficult to find all combinations of land use in more aridic environments.

Is anyone using these data to test simulation models such as Daycent or Century?

Great question, but at present I have not been approached by anyone wanting to conduct such modeling analyses. It could be a possibility for future research.