Every time we come,be ourselves.

今年冬天很冷，文章更冷．．．

在一次NBA全明星赛上解说员介绍场上球员：

Allen Iverson :skillful

Shaquille O'Neal :power

Kobe Bryant :aptitude

vince carter :dunky

...

yaoming:SIZE

敏敏这个大忙人又把担子丢给我了

原稿是: 

Separate assessment of nutrient uptake by individual plants in mixed cropping with trees is impossible without tracer techniques. The different 15N-to-14N isotope ratio of atmospheric and soil N can be used to study the contribution of biologically fixed N to the nutrition of associated trees. In most cases, the assessment of nutrient uptake distribution is an appropriate way of evaluating how to improve the transfer of biologically fixed N. Radioisotopes (e.g., 32P), stable isotopes (e.g., 15N) and rare elements (e.g., Sr)can be used to determine relative root activity distribution by applying the tracer to different soil depths or distances from trees. A broadcast application of the tracer instead of point application makes it possibe to calculate uptake values per unit area. The direct determination of nutrient pathways with such robust experiments offers considerable advantages for improving nutrient use efficiency and complementarity in multistrata agroforestry systems.

Agroforestry Systems 53: 133–140, 2001.

© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

Introduction

Complementarity of nutrient uptake is an important goal for intercropping different trees and crops, and in order to increase nutrient use efficiency and production. On the other hand, nutrient competition may decrease crop production

(Schroth et al., 2001). However, improved N nutrition and growth of a non-legume species may be achieved if it has access to biologically fixed N from an associated legume tree. Advantages or disadvantages of intercropping for plant nutrition are difficult to assess in agroforestry systems due to their spatial and/or temporal discontinuity.

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