Publisher Summary This chapter discusses the competing theories of root lifespan and reviews the evidence available to support them. New methods of root observation and analysis produce data appropriate to testing these theories, but the results till date are few and often conflicting. Tentative generalizations include a suggestion that small diameter roots with low tissue density tend to have short lifespan. Root lifespan appears to be longest in cold environments, but data are lacking for tropical species. There is a strong seasonal variation in lifespan, with roots produced in the fall surviving longest, at least in temperate climates. Species differences are difficult to quantify because of seasonal and interannual variation, but root lifespan of deciduous fruit crops seems to be shorter than those of temperate deciduous forest trees or citrus, a broadleaf evergreen. The current model of root efficiency omits some important factors that may exert control over root lifespan. Fine roots have other functions in addition to absorption, including transport of water and nutrients. Furthermore, seasonality of climate and the need for carbon and nutrient storage could constrain the root lifespan that optimizes plant fitness to differ from that which maximizes root efficiency.

The Ecology of Root Lifespan

I On the Critical N Concentration in Agricultural Crops- 1 N Uptake and Distribution in Plant Canopies- 2 Use of the Nitrogen Nutrition Index for the Analysis of Agronomical Data- II The Nitrogen Requirement of Major Agricultural Crops- 3 Grasslands- 4 Wheat, Barley, and Durum Wheat- 5 Maize and Sorghum- 6 Grain Legumes- 7 Potatoes- 8 Mixed Crops- III Management of N Nutrition- 9 Nitrogen Diagnosis and Decision Support- 10 Diagnosis Using Stem Base Extract: JUBIL Method- 11 Leaf N Content as an Indicator of Crop N Nutrition Status- 12 Radiometric Estimates of Nitrogen Status of Leaves and Canopies- 13 Concluding Remarks: N Hazards to Crops and Environment

Diagnosis of the Nitrogen Status in Crops

Advancing fine root research with minirhizotrons.

An appraisal of recent field measurements of symbiotic N2 fixation by annual legumes

Because nitrogen is one of the major elements limiting growth of plants in agrosystems, large amounts of N fertilisers have been used in the second half of the twentieth century. Chemical fertilisers have contributed to increasing crop yields and food supply, but they have induced environmental damage such as nitrate pollution and wasting fossil fuel. The use of legumes grown in rotations or intercropping is now regarded as an alternative and sustainable way of introducing N into lower input agrosystems. Here we review agricultural practices, measurement methods and biological pathways involved in N cycling. We show that plant roots interact intimately with soil microflora to convert the most abundant but relatively inert form of N, atmospheric N2, into biological substrates available for growth of other plants, through two consecutive processes; namely, N2 fixation and N rhizodeposition. In intercropping, companion plants benefit from biological fixation by legumes and subsequent transfer of N from legumes to non-legumes. This transfer from legumes to the release of N compounds by legume roots, a process named rhizodeposition, then the uptake by the companion crop. The two main rhizodeposition pathways are (i) decomposition and decay of nodules and root cells, and (ii) exudation of soluble N compounds by plant roots. The contribution of root N and rhizodeposited N to the soil-N pool is difficult to measure, particularly in the field. Firstly, root N is often underestimated because root recovery is problematic. Second, assessment of N rhizodeposition is challenging. Several 15 N labelling methods have been performed for different legume species. Rhizodeposition of N, as a percentage of total plant N, varied from 4 to 71%. The high variability of the results illustrates the need for more studies of the environmental and genetic factors influencing the amount of N rhizodeposits released by legumes under field conditions. N rhizodeposition / legumes / N2 fixation / 15 N / isotopic methods / root exudates / ecological fertilisation

/pdf/nitrogen-rhizodeposition-of-legumes-a-review-wy3i905jv2.pdf

Nitrogen rhizodeposition of legumes. A review

Legumes can transfer significant amounts of symbiotically fixed N to neighboring plants, and a putative pathway for N transfer is decomposition of fine roots (those that are not secondarily thickened) and nodules. Our objective was to quantify the amount of N in living and dead roots and nodules of alfalfa (Medicago sativa L.) and birdsfoot trefoil (Lotus corniculatus L.). These values were used with estimates of root length and nodule numbers that decomposed during a season to estimate the size of this pathway of N transfer. Birdsfoot trefoil and effectively and ineffectively nodulated alfalfa were grown in root observation boxes filled with 15 N-labeled topsoil (Typic Hapludolls). Roots and nodules were removed according to age and analyzed for total N and C [...]

Forage Legume Roots and Nodules and Their Role in Nitrogen Transfer

Growth and decay of individual roots (i.e., root turnover) is difficult to quantify because the processes occur simultaneously in a root system. Minirhizotrons are promising tools for assessing root turnover in natural and managed ecosystems. We investigated methodological problems and improvements in using minirhizotrons to estimate root turnover. Root images were collected in field plots of alfalfa (Medicaga sativa L.) and birdsfoot trefoil (Lotus corniculatus L.) through minirhizotrons installed horizontally before planting at six depths, from 7 to 70 cm, parallel to and directly below the crop rows. Forty video images (13 by 17 mm) were recorded at weekly intervals at 90° from vertical along both sides of the tubes. Images were edited into time-lapse sequences and cumulative numbers of roots that appeared and disappeared at each location were determined manually over an entire growing season. This required 29 h of labor for each replicate of each depth (a total of 80 images in one tube), with the biggest share spent on editing. Accuracy and precision of results were not decreased by using only every second image along both sides of the tube (40 images), but declined when either only one side of the tube or only 20 consecutive images on both sides were used. Reducing the recording frequency further reduced labor requirements: depending on date of root appearance and observation depth, recording intervals could be lengthened to 2 wk for the 12-cm depth and longer for greater depths without losing either accuracy or precision. Analyzing every second image and increasing the time interval between recordings resulted in projected labor savings of ≥ 70 %, while maintaining data integrity

Reducing the Cost of Estimating Root Turnover with Horizontally Installed Minirhizotrons

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Forage Legume Roots and Nodules and Their Role in Nitrogen Transfer

Reducing the Cost of Estimating Root Turnover with Horizontally Installed Minirhizotrons