| This corpus-based study investigated the developmental features of lexical richness in English compositions written by Chinese EFL students. Using both cross-sectional and longitudinal approaches, it was aimed at exploring the developmental features in four perspectives:(1) developmental features of lexical richness in terms of lexical variation, lexical density, lexical sophistication, word length and word frequency distribution; (2) the relationship between lexical richness and the writing quality; (3) the shared features and discrepancies in the development of lexical richness of L2 writings found in the cross-sectional study and the longitudinal study; and (4) the features of most overused words in the L2 writings regarding the development of lexical richness in the writings by the Chinese English major students.The data of this study came from 600 timed argumentative compositions on one topic written by 510 Chinese English majors from five universities of different types. The cross-sectional corpus was based on the 480 compositions written by 480 students from Year One to Year Four in four universities while the longitudinal corpus was composed of 120 compositions written by 30 students from Year One to Year Four in one university. They were processed, tagged and emerged to form raw corpus and tagged corpus for quantitative and qualitative studies. WordSmith5.0 Tools and Range32 were mainly used for quantitative study. Descriptive analysis, correlation analysis, one-way ANOVA test, post hoc multiple comparison test if necessary, multiple regression analysis and independent samples t-test were adopted in the quantitative study. AntConc3.2.1 was employed in qualitative study, which mainly focused on the case analysis of the features of the overused words.The present study produced a number of important findings in terms of six dimensions based on quantitative analysis. With regard to the developmental features of lexical variation, both the cross-sectional and the longitudinal studies found that the Chinese English majors made significant progress during the four years of EFL learning, while nonlinear developmental tendencies were found over the four academic years. In the cross-sectional study, plateau phenomenon was found in the writings of Year One and Year Four students, whereas plateau phenomenon only existed in Year Four in longitudinal study. In addition, cross-corpus discrepancies were found existing among freshmen, which means that writing proficiency of the freshmen in the longitudinal study was lower than that of the freshmen in the cross-sectional study.In respect of lexical density, this study found that improvement was made to some extent but not significant by the EFL learners from Year One to Year Four in both the cross-sectional and the longitudinal studies. Incongruity of the developmental tendencies was also found between the results in the cross-sectional and the longitudinal studies.As for lexical sophistication, this study found that the Chinese English majors under investigation made significant improvement in lexical sophistication across four years of EFL learning. The developmental features in both the cross-sectional and the longitudinal studies were similar. However, cross-corpus significant disparities were found in the developmental tendencies from Year One to Year Four. The seniors produced a higher percentage of lexical sophistication in the longitudinal study than that of the cross-sectional study.With respect to word length, this study found that the students overused words with two to four letters in both the cross-sectional and the longitudinal studies. From the first year to the fourth year, the developmental feature was that longer words were more frequently used with the decreasing of short wordsAs to the features of word frequency distribution, this study found that the Chinese English majors extremely depended on the high-frequency words in writing, whereas with the improvement of students'English proficiency, the use of high-frequency words decreased and accordingly the use of low-frequency words increased. Cross-corpus comparisons indicated that the developmental tendencies were similar across the four years of students'EFL learning.In terms of the relationship between lexical richness and the overall writing performance, this study found that the different dimensions of lexical richness had different influence on the overall English writing performance. Lexical variation and lexical sophistication played a more important role in the writing performance of the juniors and the seniors than in that of the freshmen and the sophomores. In addition, lexical density and word length were found to have little impact on the overall writing performance. The second 1,000 most frequent word families had some influence on the overall writing performance of the freshmen in both the cross-sectional and the longitudinal studies.Qualitative analysis showed that the most overused words were function words and high-frequency lexical words except for the topic-related words in comparison with LOCNESS. It was found that the first and second personal pronouns, the modal auxiliary, the high-frequency lexical word and the multi-function word were overused in the English compositions written by the Chinese English majors compared with the NSs'writings. The developmental tendencies of these words in frequency were similar to the vocabulary use of the NSs over the four years of EFL learning.Based on the results found in quantitative and qualitative analyses, this study made attempts to interpret and explain why the results were produced and discussed the main findings by comparing them with the results of previous studies and by referring to existing theories of second language acquisition, discourse studies and cultural studies. The aspects discussed include the overuse of high-frequency lexical words in writings by the Chinese English majors, the plateau phenomenon in lexical variation, models of developmental features of lexical richness, unbalanced effects of different dimensions of lexical richness on the overall writing performance, and the developmental features of the overused word cases.This study concluded that these findings would provide pedagogical implications for the purpose of facilitating the vocabulary teaching in L2 writing. Tentative suggestions discussed include adopting reward system in evaluating L2 writings, turning the passive vocabulary into active vocabulary, integrating vocabulary teaching into writing instruction, overcoming the vocabulary plateau problem, reducing style-interference, developing vocabulary learning strategies, launching corpus-based vocabulary teaching, and applying user-friendly software to vocabulary teaching in L2 writing. Limitations of this study and recommendations for future studies were finally presented.
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