The components of property and equipment were as follows:


(in millions)	Useful Lives	December 31, 2021		December 31, 2020

Land		$	225	$	236
Buildings and equipment	Up to 30 years	4,344		4,006
Wireless communications systems	Up to 20 years	57,114		49,453
Leasehold improvements	Up to 10 years	2,160		1,879
Capitalized software	Up to 10 years	18,243		16,412
Leased wireless devices	Up to 19 months	3,832		6,989
Construction in progress	N/A	3,703		4,595
Accumulated depreciation and amortization		(49,818)		(42,395)

Property and equipment, net		$	39,803	$	41,175

Total depreciation expense relating to property and equipment and financing lease right-of-use assets was $15.2 billion, $13.1 billion and $6.5 billion for the years ended December 31, 2021, 2020 and 2019, respectively. These amounts include depreciation expense related to leased wireless devices of $3.1 billion for each of the years ended December 31, 2021 and 2020 and $543 million for the year ended December 31, 2019.

We capitalize interest associated with the acquisition or construction of certain property and equipment and spectrum intangible assets. We recognized capitalized interest of $210 million, $440 million and $473 million for the years ended December 31, 2021, 2020 and 2019, respectively.

Asset retirement obligations are primarily for certain legal obligations to remediate leased property on which our network infrastructure and administrative assets are located.

Activity in our asset retirement obligations was as follows:


(in millions)	Year Ended December 31, 2021		Year Ended December 31, 2020

Asset retirement obligations, beginning of year	$	1,817	$	659
Fair value of liabilities acquired through Merger	—		1,110
Liabilities incurred	54		16
Liabilities settled	(173)		(40)
Accretion expense	62		55
Changes in estimated cash flows	139		17

Asset retirement obligations, end of period	$	1,899	$	1,817

Classified on the consolidated balance sheets as:

Other current liabilities	$	216	$	14
Other long-term liabilities	1,683		1,803

The corresponding assets, net of accumulated depreciation, related to asset retirement obligations were $613 million and $912 million as of December 31, 2021 and 2020, respectively.

In connection with the continuing integration of the businesses following the Merger, we evaluated the long-term billing system architecture strategy for our postpaid customers. In order to facilitate customer migration from the Sprint legacy billing platform, our postpaid billing system replacement plan and associated development will no longer serve our future needs. As a result, we recorded a non-cash impairment $200 million related to capitalized software development costs for the year ended December 31, 2020, all of which relates to the impairment recognized during the three months ended June 30, 2020. The expense is included in Impairment expense on our Consolidated Statements of Comprehensive Income. There were no impairments recognized for the years ended 2021 and 2019.