The "Giant Panda" in the farmland: We finally know how old it is!

Produced by: Science Popularization China

Author: Chen Dongfeng, Li Chengbao (Nanjing Institute of Soil Science, Chinese Academy of Sciences)

Producer: China Science Expo

Black soil is no stranger to us. Soil is a collection of mineral particles containing organic matter on the surface of the land, while black soil is soil that is rich in organic matter (carbon) and has a gray-black color.

The biggest feature of black soil is that it contains rich organic carbon . If we can accurately measure the organic carbon initially produced in the soil layer and its subsequent accumulation and evolution, we will be able to determine the formation age and evolution rate of the soil, which is of great significance for the sustainable utilization and management of black soil resources . In addition, as the key to China's food security, the effective protection of black soil resources is crucial to the construction of China's great granary.

So how is the organic matter in black soil produced? How does it continue to accumulate and evolve after it is produced? Don't worry, let's take a look together.

Black soil age measurement? More complicated than you think

Soil is an extremely complex open system , in which the exchange, migration and transformation of substances are always present. Such complex physical, chemical and biological characteristics make it very difficult to accurately define the age of soil formation.

In order to more accurately measure and determine the soil age, researchers from the Nanjing Institute of Soil Science, Chinese Academy of Sciences and Nanjing Institute of Geography and Limnology, based on extensive surveys of dozens of profiles in the Northeast Black Soil Region, studied the carbon fragments that appeared below the bottom boundary of the black soil layer and the deeply buried black soil layer. They also conducted carbon 14 and OSL dating on three black soil profiles that developed naturally under smooth terrain conditions.

Topography and geomorphology map of Northeast Black Soil Region

(Image source: Chinese Academy of Sciences)

Through this research, Chinese scientists have effectively solved the difficult problem of how to accurately define the age of black soil, and for the first time clarified that the initial age range of black soil formation in typical black soil areas in Northeast China is between 16.9 and 12.6 thousand years ago, much earlier than the previously generally believed early and middle Holocene (10 to 5 thousand years).

Carbon-14 dating is the radiocarbon dating method, also known as Carbon-14 dating. It is a measurement method that calculates the approximate age of a sample based on the decay degree of carbon-14 (the half-life of carbon-14 is 5730 years). This method is usually used to determine the age of paleontological fossils, ranging from 58 to 62 thousand years. The radiocarbon method is the most accurate, widely used and mature method in the Quaternary geological dating method, and is widely used in geological, environmental and archaeological research since 50 thousand years ago.

OSL (Optically stimulated luminescence) dating is based on the same principle as isotope dating: there is a functional relationship between the total amount, rate and time. If the total amount and rate are known, the time (decade) can be calculated. All buried geological bodies and archaeological samples containing minerals such as quartz or feldspar that have been fully exposed to sunlight (natural light) can be used for luminescence dating, such as loess, dune sand, ancient pottery fragments, rapidly uplifted bedrock, etc.

The researchers used the above two methods to measure the black soil in Northeast China. The results showed that the OSL age of the base of the black soil layer (14 to 13.5 thousand years) is much older than the carbon 14 age (9 to 7 thousand years). It can be inferred that the OSL age represents the age of the organic carbon first produced in the black soil, while the carbon 14 age (average retention time of soil organic carbon) of the whole soil (later accumulated black soil layer) greatly underestimates the formation age of the black soil.

Black soil profile

(Image source: Chinese Academy of Sciences)

This inference is consistent with the Quaternary geological environment background. The OSL age corresponds to the late glacial warm period (approximately 14.7-12.7 thousand years ago) . The regional climate since the Last Glacial Maximum has experienced rapid warming and humidification during this period, which significantly increased biomass and provided favorable conditions for the cumulative evolution of soil organic matter.

This is also confirmed by the age of the buried black soil layer. Based on the huge changes in carbon 14 and OSL ages, the sedimentation process of soil parent materials and regional historical environment, the researchers proposed a new model for the formation of black soil - the accumulation model, which believes that the formation of the deep black soil layer in Northeast China is the result of the accumulation of organic matter and the continuous accumulation of mineral clay particles. This model not only reveals the formation mechanism of the deep black soil layer, but also explains why the soil organic carbon carbon 14 age seriously underestimates the formation age of black soil.

Depth variation characteristics of carbon 14 age and OSL age of organic matter components in black soil reveal the accumulation pattern of black soil formation

(Image source: Chinese Academy of Sciences)

Protecting the black soil, using advanced technology

Accurate research on the accumulation and evolution of organic matter in black soil will become an effective way to reveal how to fertilize black soil and improve soil productivity, and should be the first direction of current soil science research.

In 2021, the Chinese Academy of Sciences launched and implemented the "Black Soil Granary" scientific and technological campaign, which is a major initiative to study and reveal the accumulation and evolution laws of black soil organic matter. A research team from the Nanjing Institute of Soil Science is the backbone of this campaign.

The task of fertilizing regional black soil and improving its productivity over a large area is very arduous. Soil organic matter , as the core indicator of the intrinsic productivity of cultivated land, is an important factor affecting the quality of black soil.

Spatial distribution map of soil organic matter content in cultivated land in the Northeast Black Soil Region

(Image source: Chinese Academy of Sciences)

At present, methods such as increasing the application of organic fertilizers, planting green manure, returning straw to the fields, using fertile microbial agents, and conservation tillage can all effectively increase organic matter, but the process of converting added organic matter into soil organic matter is very slow.

In order to solve the problem of speed, the research team of Nanjing Institute of Soil Science conducted a lot of investigations and studies on the rapid and continuous increase of organic matter content in black soil and the cultivation of healthy soil, and basically established an effective carbon-enhancing fertile soil technology system.

This technical system adopts comprehensive agricultural technical measures, applies as much farm organic fertilizer fermented from human, livestock and poultry manure as possible, combines green manure with straw and returns it to the field, and applies microbial agents containing Bacillus megaterium to accelerate the decay and decomposition of organic matter, form soil humus, and increase the organic matter content of black soil. It also adopts deep plowing, turning and harrowing and other tillage measures to provide technical support for effectively fertilizing black soil and improving soil productivity.

Farmyard manure in the field

(Photo source: Veer Gallery)

In response to the problems of black soil becoming thinner, thinner and harder, the national science and technology battle team is currently conducting integrated sky-ground monitoring and early warning research to accelerate the realization of intelligent operations in black soil production.

Land plowing, crop sowing and planting, and fertilization are now automated and unmanned. Through satellite positioning, smart weeders can accurately remove weeds from field crops and mechanize weeding. This eliminates pollution caused by the use of herbicides and protects the arable land environment.

A new generation of unmanned intelligent agricultural machinery

(Photo source: Xinhua News Agency)

In addition, researchers are also continuously conducting research and development on soil degradation control technology, comprehensively planning and building shelterbelts in black soil areas, and planting perennial forage grasses on windy hills and slopes to prevent wind erosion and soil and water loss, which plays an important role in accumulating organic matter and protecting the black soil layer. In order to realize the comprehensive protection and utilization of black soil, it is necessary to vigorously develop various types of smart farming and management machinery to meet the urgent needs of rural revitalization and agricultural modernization.

Conclusion

In the black soil area of ​​Northeast China, the deep soil layers are all black, which is strong evidence that the black soil is rich in organic carbon. Accurately measuring the age of black soil and proposing an accumulation model are of great significance for understanding the formation process and mechanism of black soil resources, evaluating the sustainability of black soil resources, and determining the age of soil. It can be imagined that if the accumulation and evolution of organic carbon in black soil can be understood, and then a technology that can increase the organic carbon content of general soil to the level of general black soil can be developed, the capacity of the soil carbon pool will be greatly increased. This will promote the early realization of my country's "dual carbon" goals and will undoubtedly be a major contribution to global climate change.

References:

[1]Edited by Nanjing Institute of Soil Science, Chinese Academy of Sciences, 1978, Soils of China, Science Press.

[2]Zhang G., Long H., and Yang F. (2023). Understanding the formation time of black soils. The Innovation Geoscience 1(1), 100010