Quantitative study of atmospheric nitrogen uptake and NH3 interception in soil emission in rice canopy in Nanjing soil...

The plant canopy is a key place for the exchange of substances and energy between crops and the outside world such as water, air and heat, and due to the two-way exchange between the canopy and the atmosphere, NH₃ and_NO2 will also affect some key processes
of the nitrogen cycle in farmland.
For example, atmospheric reactive nitrogen tends to enter the soil through wet and dry deposition, increasing soil nitrogen supply
.
In fact, gaseous_NH3 or_NO2, etc.
, may also be absorbed directly into crops by the canopy
before atmospheric deposition nitrogen reaches the soil of the field.
Previous studies have only quantified the agronomic effects of atmospheric deposition nitrogen from the perspective of the total amount of available nitrogen collected at the interface, and paid little attention to the contribution
of crop canopy to atmospheric active nitrogen retention and absorption.
In addition, NH₃ volatilization is an important pathway of nitrogen loss in farmland, and in situ observation of it is usually by box method or micrometeorological method
.
Since most of the studies using the compartment method do not consider crops, but only monitor NH3 emissions from the soil surface, the results are very different from the_NH3 volatilization measured by the soil-crop system-based micrometeorological method, and it is not clear
whether this difference is caused by crop canopy retention 。 Based on this, the research team of Zhao Xu of Changshu Agroecology Experimental Station, Nanjing Institute of Soil, Chinese Academy of Sciences, selected the rice field of the Taihu Plain as the main research object, and answered the question "Can the rice canopy directly absorb atmospheric reactive nitrogen as nitrogen nutrients?" through continuous attempts, using ingenious field experimental design and continuous observation.
and "Can the rice canopy intercept the discharge of NH₃ from the soil surface after fertilization?" "Two questions
.

There are two difficulties in the quantitative study of atmospheric active nitrogen uptake by crop canopy: first, the sources of nitrogen in farmland are complex, including soil nitrogen, fertilizer nitrogen, irrigation water nitrogen, biological nitrogen fixation, rainfall dust nitrogen, and direct uptake of atmospheric nitrogen by the canopy; Second, the nitrogen in the farmland can be directly lost through volatilization, runoff, leaching and other ways, and the nitrogen absorbed by crops can also be discharged
to the environment through the plant channel.
In response to these problems, the research team designed a rice growth system (ZL202120780243.
7) that can study the uptake of atmospheric reactive nitrogen by the canopy during the entire rice growth period under field conditions: First, utilize ¹⁵ The N-labeled nutrient solution is the only source of nutrients absorbed by rice roots, and then the interference of other nitrogen sources and the loss of isotope-labeled nitrogen in the nutrient solution are eliminated and the loss of isotope-labeled nitrogen in the nutrient solution is avoided to the greatest extent by controlling the pH of the nutrient solution lower than 6, sealing the growth box to keep the dark environment, and sheltering the rain to avoid the loss of isotope-labeled nitrogen in the nutrient solution
, thus ensuring that the whole growth period of rice in the research system can only be obtained through the root absorption of fertilizer nitrogen and the canopy absorption of atmospheric active nitrogen 。 Field operation found that the growth law of rice during the whole growth period in this system was basically consistent with the field situation, which was well representative.
The nitrogen recovery rate of labeled fertilizer at the rice jointing stage was 98%, indicating that the direct loss of fertilizer nitrogen in the system was very small, and the interference to the results was very small
.
On this basis, the apparent amount of atmospheric active nitrogen absorbed by rice during the whole growth period was calculated by plant source nitrogen differentiation as 61 kg N ha-1, but the labeled nutrient liquid nitrogen absorbed by the root system in rice lost another 27 kg N ha-1 through the plant channel during the reproductive growth period, so the net absorption of atmospheric active nitrogen in rice during the whole growth period was 34 kg N ^ha-1
。 This study breaks through the traditional view that farmland nutrient supply depends on root absorption, and finds that the direct absorption of atmospheric reactive nitrogen by the canopy in the field environment is also an effective way for crop nutrient acquisition, which provides a new perspective
for the research on farmland nitrogen budget and optimal management under the increase of nitrogen from environmental sources.

In response to the question "Can the crop canopy intercept the discharge of NH^₃ from the soil surface after fertilization?" The team used a large-scale closed-room pumping method to simultaneously monitor NH3 emissions from the soil surface and soil-crop system during the rice season by wrapping and not wrapping plants, so as to distinguish and quantify the impact
of rice canopy on soil_NH3 emissions.
It was found that the box method to determine whether NH 3 discharge included crops had a greater impact on the results, with a difference of more than 1 times, especially from the middle to the ripening stage of rice tillering, indicating that rice canopy had a significant retention effect
on soil surface discharge NH₃.
The continuous monitoring of soil and soil-crop system NH 3 emissions at the peak of rice tillering showed that NH 3 interception in rice canopy mainly occurred during the day, and the greater the soil NH₃ discharge flux, the more obvious
the retention effect.
Observations of the whole rice growing season showed that the soil NH₃ discharge was large after top dressing in the early stage of tillering (the time when farmers in the study area were accustomed to tiller fertilizer), but due to the small plant, the canopy interception was not obvious.
If the application time of tiller fertilizer is appropriately extended to the middle stage of tillering (about 2 weeks), the interception effect of canopy on soil surface NH 3 emissions can be significantly improved, and the NH 3 emissions of soil-crop system system can be reduced by 71% and the NH₃ volatile loss of fertilizer nitrogen in the whole rice season can be reduced by 31%.
Appropriately postponing the application time of tiller fertilizer not only reduces the volatilization loss of fertilizer_{NH 3}, but also improves the matching between fertilizer nitrogen supply and crop nitrogen demand, increases the effective ear number and ear grain number of rice, increases rice yield by 9-15%, and increases the apparent nitrogen fertilizer utilization rate in the current season by 45-50%.

This study obtained direct evidence of crop canopy interception soil NH 3 emissions, which not only provides an important basis for achieving farmland weight loss and emission reduction through nitrogen fertilizer management, but also provides a new perspective
for the scientific monitoring of NH 3 volatilization in farmland and the accurate assessment of fertilizer NH₃ volatilization loss.
Under normal circumstances, the determination of soil NH 3 emissions based on the box method (excluding crops), without considering the impact of crop growth and canopy, may lead to a certain degree of overestimation of NH 3 volatile losses in farmland, so this issue
should be paid attention to in future research on monitoring and evaluation of NH₃ volatile losses in farmland.

Recently, the above research results have been published online in the Journal of Agricultural and Food Chemistry (cover article; Discovery of a significant amount of canopy uptake of atmospheric reactive N during rice growth) and the European Journal of Agronomy
.
Dr.
Tian Yuhua is the first author of the above paper, and researcher Zhao Xu is the corresponding author
of the paper.
The above research work was supported
by the Outstanding Member Fund of the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Y201956), the "13th Five-Year Plan" and the "14th Five-Year Plan" Key R&D Program of the Ministry of Science and Technology (2017YFD0200104, 2021YFD1700805).

Links to papers: 1, 2