May 18, 2021 at 10:43 am | Updated March 14, 2022 at 1:30 pm | 7 min read
Increasing yield has been and continues to be the holy grail in crop breeding. Aided by minirhizotrons, root scanning is increasingly used to understand underlying causes, such as root growth response to management practices. This approach could help to achieve higher yields without increasing nitrogen inputs. In one such attempt, a team of scientists in China decided to investigate the effect of increasing the number of nitrogen top dressings on wheat.
Rice-Wheat Rotations Are Not Always Optimum
In China, about 95% of wheat is grown during the winter. Of this, 75% is grown in a multi-cropping system along with rice, the other predominant crop of the country. When rice is grown before wheat, it can affect the soil condition by reducing residual nutrient availability, especially nitrogen levels. Rice cultivation can also create waterlogging stress when it precedes wheat.
Dryland wheat production produces more yield than rice grown in wetland or stagnant water. Nitrogen levels can, however, be critical in dryland rice-wheat systems, too.
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So far, one of the simplest options has been to increase nitrogen treatments in an attempt to improve wheat yield. However, the amount of nitrogen applied cannot be increased any further, as the current levels are already high. Increasing nitrogen treatments will add to farming costs without increasing yield and it will increase the environmental impact of food production, in terms of carbon emissions and pollution of land and water.
Zooming in on Unknowns: Fertilizer Timing
The influence of the timing and frequency of nitrogen treatments is still unknown for the rice-wheat systems in China. Hence, a team of agricultural scientists in China decided to study the crop physiology of wheat when it was fertilized at different times and with a varying dosage of nitrogen. They wanted to see how root behavior, which is instrumental in the absorption of nutrients from the soil, reacts to varying doses and affects nitrogen-use efficiency.
The scientists, Yang, Liu, Geng, Zhang, Yin, and Wang, also evaluated the rice rotation (RW) by comparing it with a second combination, where winter wheat is grown after summer soybean (DW).
The experiment took place in the Jianghan Plain, Hubei Province, which receives ample rainfall, so the crops were not likely to suffer from drought stress even though they were not irrigated. They applied the same amount of fertilizers (180 kg N ha−1) but varied the dosage. The three nitrogen treatments given to wheat in both the rotations, as urea, for two subsequent years, were as follows:
- CK was the control and got no fertilizers,
- M1 plants got two doses, a basal and one top dressing, of 90 and 90 kg N ha−1,
- and M2 plants were given three doses, one basal and two top dressing, of 60, 60, and 60 kg N ha−1.
Wheat sowing rate, and all other nutrients and treatments, against pests and diseases were the same for all the trials.
At the end of each season, plants from the middle of each plot were selected for data collection for yield parameters, such as total grain yield, spike number, 1000 kernel weight, and the average number of kernels in 30 spikes. Also, the shoot biomass of twenty plants was separated into the stem, sheath, flag leaf, other leaves, hull and rachis, and the grain components to estimate nitrogen content by the Kjeldahl method.
Root Dynamics Observations By Minirhizotron
To observe root dynamics, the agricultural scientists needed to quantify root length density at various soil depths. Because root research requires examining factors that necessarily lie underground, it can present unique challenges. Digging a root system out can stress or destroy the plant, and lead to a loss of finer roots, affecting the accuracy of the data. Rhizotron boxes can be useful, but require dedicated construction, which is often not plausible – especially in-situ.
Therefore, the agricultural scientists decided to use a minirhizotron for non-destructive root scanning. The CI-600 In-Situ Root Imager, manufactured by CID Bio-Science, is suitable for monitoring root growth. The scientists installed one-meter deep clear plastic soil tubes, with an internal diameter of 50.8 mm, before sowing. The tubes were inserted at an angle of 45 degrees before sowing the crop and covered with a cap to prevent soil, rain, and roots from entering. The roots of wheat plants grow around the tubes, undisturbed, throughout the season. The tops were also covered by thermal foils to prevent light from getting into the tube and warming it.
At the end of each season, the scientists estimated the root densities with the minirhizotron, which has a scan head that can be rotated 360 degrees. The scientists inserted the minirhizotron into the root tubes, with the help of an automatic indexing handle, to accurately measure roots in three zones (0-20 cm, 20-40 cm, and 40-60 cm depth), taking into consideration the 45 degree angle of the root tubes.
The root scanning was rapid, allowing the agricultural scientists to quickly collect high-resolution images of the entire root system. The accompanying RootSnap! software made it easy to identify even the fine roots from the surrounding soil, so no roots are missed in the data. They calculated the root length from the images using the WinRhizotron software.
Soil samples taken at every 20 cm layer, up to a depth of two meters, were analyzed for nitrate accumulation. This was done at two places in the field. Five other soil samples were used to estimate soil moisture content at 0-20 cm and 20-40 cm soil layers.
Frequent Fertilization is Better
The agricultural scientists found that the yields of wheat grown after rice or soybean were comparable. The control without fertilization had the lowest yield. Increasing the frequencies of top-dressing improved yield significantly in both rotations for wheat. Three doses of N increased wheat yield by 11% in the soybean rotation and by 15% in the rice rotation. This can be seen in the 15% rise in spike numbers in soybean rotations and 13% in rice rotations.
This occurs even though 1000 kernel weight is not different in the two and three-dose treatments. It is the higher number of kernels per spikes in the three-dose systems that improve yield. The scientists would like to conduct more studies with varieties that have a higher kernel number to confirm this insight.
Wheat crops in rice rotations had a lower nitrogen uptake than in soybean rotations. Increasing fertilizer frequency improved the uptake of nitrogen. The nitrogen accumulation in soybean-wheat was 24% higher in two-dose treatments and 33% higher in the three-dose treatments. This difference was significant in soybean-wheat rotations, though it was not significant in the rice-wheat rotations. Of all the aboveground parts, the difference in nitrogen accumulation was highest in grains; see Figure 1.
The difference in nitrogen uptake is a reflection of root growth in wheat plants in the two rotations. In both the crop rotations, the shallowest soil layer had 50% of the root density. At deeper soil layers, crop rotations influence root growth. Wheat grown after rice (RW) had only 4-8% of roots below 40 cm, while wheat grown after soybean had 16-18% root density in the same soil depths; see Figure 2. The scientists think that the soybean roots, like those of other legumes, increase soil pores in the subsoil, making it looser and easier for subsequent crops to grow deeper roots.
Increasing doses from two to three times, even when the total quantity was the same, increased the root length density (RDL) in both the crop rotations. As Figure 2 shows, any type of fertilization is better than the control.
The root growth, in turn, reflects the nitrate distribution in the soil. The nitrogen in the soil was higher in the soybean rotations than rice rotations by 75% in the two-dose system and 63% in the three-dose system.
In soybean rotation, there was more nitrogen in the shallow and deeper soil layers than in rice rotations. The higher nitrogen accumulation in the soil can be explained by biological nitrogen fixation by the leguminous soybean.
The scientists also argue that more compact and wetter soils in rice could lead to lower oxygen. This can cause denitrification and loss of nitrogen in deeper soils. The scientists also speculate that the higher water capacity at a shallow depth in the soil for rice allows it to retain more nitrogen because less rainwater seeps to lower levels and leeches nitrogen with it.
The agricultural scientists report that increasing the top-dressing in rice-wheat rotations can narrow the yield gap seen in soybean rotations. This, however, does not improve nitrogen use by wheat plants, as they are still affected by the influences of rice cultivation that limits nitrogen percolation to lower soil depths.
They also recommend sowing wheat after rice, without tillage, to take advantage of soil compaction by rice harvesting machines. This decreases plant density, but the resultant greater spacing improves leaf area and photosynthesis and, as a result, the spike and kernel numbers.
By including prior land use as a factor, the scientists were able to see the effect that rice has on wheat. The choice of soybean was the perfect foil to rice, given its deep root system. However, biological nitrogen fixation by soybean could have influenced the interpretations of the results. Including a non-legume crop rotation could have helped correct for this. Regardless of this one confounding factor, the scientists have opened many avenues for improvement of rice varieties, like frequency of top dressing, the role of compact soil, and focusing on yield parameters such as the number of kernels. Overall, this study was critical in providing new directions and insights into winter wheat crop physiology under Chinese agricultural conditions.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
Liu, K., He, A., Ye, C. et al. Root Morphological Traits and Spatial Distribution under Different Nitrogen Treatments and Their Relationship with Grain Yield in Super Hybrid Rice. Sci Rep 8, 131 (2018). https://doi.org/10.1038/s41598-017-18576-4
Yang, R., Liu. K., Geng, S., Zhang, C., Yin, L., & Wang, X. (2021). Comparison of early season crop types for wheat production and nitrogen use efficiency in the Jianghan Plain in China. PeerJ 9: e11189 https://doi.org/10.7717/peerj.11189
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