Fixing C in a changing climate: a new N challenge?
28 June 2021 - Soil Carbon Check
The Paris Climate Agreement aims to increase the carbon content of soil by 4 per mille (0.4 per cent). This must be done by increasing the organic matter in the soil. But adding organic matter also adds nitrogen to the field which can be released - sometimes very haphazardly - as a result of mineralisation. Building up organic matter while keeping nitrogen levels under control takes some working out.
Soil fertility is declining; the soil is becoming exhausted!
For many years, the message always seemed to be that soil fertility was declining at an alarming rate. Now we can see that the amount of plant available phosphate is indeed in decline1) and that the quantity and quality of soil life may also be changing. But we can’t be sure about this because soil biological indicators have not been monitored routinely for very long.
On average, the organic matter content on mineral (sand and clay) agricultural soils certainly does not seem to be declining2). In fact, this was confirmed in a recent study which shows that the organic matter content of mineral soils in the 0-30 cm layer was 4.0% in the 1990s and 4.1% in 20183).
Farmers, growers and advisors have been able to compensate for the decline in the supply of organic matter via animal manure by making operational choices (such as choice of crops, green manure crops/catch crops or tillage). This is good news, because organic matter is perhaps the most important indicator of soil fertility, and it is easier to achieve optimum food production on plots with higher soil fertility.
Paris Climate Agreement
Organic matter therefore plays a major role in our food production, but it is also important for our climate. Carbon (C) is one of the main components of organic matter, and in the 2015 Paris Climate Agreement it was agreed that steps would be taken to limit climate change by fixing more C in the soil, ideally 4 per mille (0.4% C) more per year (www.4p1000.org).
Soils naturally contain large amounts of C. On a global scale, about 1,500 petagrams (Pg; 1 Pg = 1015 grams) of C are fixed in the uppermost metre of the soil. That is three times as much as in the above-ground biomass and twice as much as in the atmosphere (as CO2). The soil is therefore quite important.
If you convert this figure of 4.1% organic matter on mineral soils into kilograms per hectare, you arrive at 150,000 kg organic matter per hectare (0-30 cm). On average, organic matter consists of around 50% C, i.e. 75,000 kg C per hectare (equivalent to almost 250,000 kg CO2) in the upper layer of the soil. A 0.4% increase would mean 600 kg organic matter more per hectare then what is currently supplied. This is equivalent to the build-up phase of a grass sward, or an extra 5 tonnes of compost or 20 tonnes of cattle manure per hectare, for example4). The product (such as compost or manure) or the method (tillage) we would use to achieve this increase is an important point for consideration.
Soil life and nitrogen
This is because organic matter is not only made up of C but also of N (along with sulphur (S) and phosphorus (P)). Soils with 75,000 kg C per hectare contain about 5,000 kg organically bound N per hectare (assuming C/N 15). Any product that increases organic matter (crop residues, green manures, catch crops, compost, animal manure) also contains N, (because organic matter consists of C, N, S and P).
The N does not stay where it is, thanks to the organisms present in the soil. These soil organisms are useful for a wide range of processes, but they need food, and organic matter is their food source. Soil organisms break down organic matter, releasing its components (mineralisation). It is a continuous process of building up and breaking down. The net result is a build-up of organic matter (and therefore fixing C), but C (as CO2) and N is also released.
Climate and nitrogen
In recent years we have been seeing the following. The hot summers have kept the soil warm, but it has also been very dry. Soil organisms becomes less active in drought conditions, so little, if any, mineralisation of the soil takes place, and no N is released. When it subsequently starts raining, or if the soil is irrigated, the soil organisms become very active and eat the organic matter (i.e. including organic N), resulting in a sudden release of large amounts of N. This process can take place on all soils, whether agricultural or natural.
Nature that is sensitive to N needs poor soil that will need to be kept poor by removing organic matter (and therefore C and N). This means that the other soils in the Netherlands will have to ‘grow’ by more than 4 per mille per year. These will mainly be agricultural soils. As the organic matter content in these soils increases, they will become less drought-prone and easier to cultivate, they will bind more cations (Ca, Mg, K) and will act as a food source for soil organisms. However, they will potentially also mineralise more N than they do at present, often at inopportune moments (for example, after modest rainfall just after harvesting), unless we use more stable sources of organic matter such as processed manure, compost, biochar and straw residues. It is therefore important to have more control over the properties of organic products (and the variation within products).
The agricultural sector has so far maintained the organic matter content of the soil, possibly precisely because of the reports of deteriorating soil quality. This has not yet been achieved in the countries around us. However, a target increase of 4 per mille or more will once again require the sector to do a lot of calculating as well as needing space to apply the products.
